In the first subfamily, Natalinae, which is exclusively tropical American, the other upper incisors are separated from one another and from the canines; palatine processes of the premaxillae are at least partially developed; and the dental formula is i. 2/3, c. 1/1, p. {2 or 3}/3, m. 3/3. In general appearance these bats recall the more typical Vespertilionidae, although the form of the muzzle is suggestive of the Mormopsinae among the Phyllostomatidae. Again, while the form of the skull is vespertilione, the relation of the vomer to the front end of the premaxillae is of the phyllostomine type. The molars and incisors are likewise vespertilione, whereas the premolars are as distinctly phyllostomine. Finally, while the third, or middle, finger normally has two phalanges, as in typical Vespertilionidae, the second of these is elongated and in Thyroptera divided into two, as in Phyllostomatidae.



The first two genera, Furipterus and Amorphochilus, each have a single species, the latter being distinguished from the former by the wide separation of the nostrils and the backward prolongation of the palate. In both the crown of the head is elevated, the thumb and first phalange of the middle finger are very short, and the premolars are 2/3. The same elevation of the crown characterizes the genera Natalus and Chilonatalus (fig. 17), in which the premolars are 3/3: in general appearance these bats are very like the Old World vespertilionine genus Cerivoula, except for the short triangular tragus. Lastly, Thyroptera includes two species distinguished by an additional phalange in the middle finger and by accessory clinging-organs attached to the extremities. In Thyroptera tricolor, i. 2/3, p. 3/3, from Brazil, these have the appearance of small, circular, stalked, hollow disks (fig. 18), resembling miniature sucking-cups of cuttle-fishes, and are attached to the inferior surfaces of the thumbs and the soles of the feet. By their aid the bat is able to maintain its hold when creeping over smooth vertical surfaces.



The second or typical subfamily, Vespertilioninae, includes all the remaining members of the family with the exception of the aberrant Molossinae. The upper incisors are in proximity to the canines; the premaxillae widely separated; the ears medium or large; the dental formula is i. 2/3 (or 1/3), c. 1/1, p. 3/3 (2/3, 2/2, or 1/2), m. 3/3; and the fibula very small and imperfect. All the members of this large cosmopolitan group are closely allied, and differ chiefly by external characters. They may be divided into subgroups. In the first of these, the Plecoteae, of which the long-eared bat (Plecotus auritus) is the type, the crown of the head is but slightly raised above the face-line, the upper incisors are close to the canines, and the nostrils are margined behind by grooves an the upper surface of the muzzle, or by rudimentary nose-leaves; the ears being generally very large and united. Of the six genera, Plecotus, with i. 2/3, p. 2/3, has three species:—one the long-eared European bat referred to above; P. macrotis, restricted to North America, is distinguished by the great size of the glandular prominences of the sides of the muzzle, which meet in the centre above and behind the nostrils; the third species being also American. The second, Barbastella, with i. 2/3, p. 2/2, distinguished by its dentition and by the outer margin of the ear being carried forwards above the mouth and in front of the eye, includes the European barbastelle bat, B. barbastellus, and B. darjelingensis from the Himalaya. Otonycteris, i. 1/3, pm., 1/2, connecting this group with the Vespertilioneae, is represented by O. hemprichii, from North Africa and the Himalaya, and an Arabian species. The next two genera are distinguished by the presence of a rudimentary nose-leaf: Nyctophilus, i. 1/3, p. 1/2, with three species from Australasia; and Antrozous, i. 1/2, p. 1/2, distinguished from all the other members of the subfamily by having but two lower incisors, and from other Plecoteae by the separate ears; the two species inhabit California. The sixth genus, Euderma, is also represented by a Californian species.





The second group Vespertilioneae, with about thirteen genera, includes the great majority of the species; and a large number of these may be classed under Vespertilio, which is divisible into subgenera, differing from one another in the number of premolars, and often ranked as separate genera. One group is represented by V. (Histiolus) magellanicus, a species remarkable for its extreme southern range, its relatives being also South American. A second group, with p. 1/2, includes the British serotine, V. (Eptesicus) serotinus, of Europe and northern Asia, and represented in North America by the closely allied V. (E.) fuscus. In the typical group, which includes the Old World V. murinus, one species, V. borealis, ranges to the Arctic circle. The European noctule, V. (Pierygistes) noctula, and Leisler's bat, V. (P.) leisleri, represent another group; and the common pipistrelle, V. (Pipistrellus) pipistrellus, yet another, with p. 2/2. The only other group that need be mentioned is one represented by the North American V. (Lasionycteris) noctivagans, with p. 2/3. The African Laeephotes, the Chinese Ia, and the Papuan Philetor are allied genera, each with a single species. Chalinolobus and Glauconycteris have the same general dental character as Vespertilio, but are distinguished by the presence of a lobe projecting from the lower lip near the gape; the former, with p. 2/2, is represented by five Australasian species, one of which extends into New Zealand; while the latter, with p. 1/2, is African. The species of Glauconycteris are noticeable for their peculiarly thin membranes traversed by distinct reticulations and parallel lines. Scotophilus, with i. 1/3, p. 1/2, includes several species, restricted to the tropical and subtropical regions of the eastern hemisphere, though widely distributed within these limits. These bats, though approaching certain species of Vespertilio in many points, are distinguished by the single (in place of two) pair of unicuspidate upper incisors separated by a wide space and placed close to the canines, by the small transverse first lower premolar crushed in between the canine and second premolar, and, generally, by their conical, nearly naked, muzzles and thick leathery membranes. S. temmincki is the commonest bat in India, and appears often before the sun has touched the horizon. S. gigas, from equatorial Africa, is the largest species. Nycticejus, with the same dental formula as Scotophilus, is distinguished, by the first lower premolar not being crushed in between the adjoining teeth, and the comparatively greater size of the last upper molar. It includes only the North American N. humeralis (crepuscularis), a bat scarcely larger than the pipistrelle. The hairy-membraned bats of the genus Lasiurus (Atalapha), with i. 1/3, p. 2/2 or 1/2, are also limited to the New World, and generally characterized by the interfemoral membrane being more or less covered with hair and by the peculiar form of the tragus, which is expanded above and abruptly curved inwards. In those species which have two upper premolars the first is extremely small and internal to the tooth-row. The genus, which is divided into Lasiurus proper and Dasypterus, is further characterized by the presence of four teats in the female, and by the general production of three or four offspring at a birth. Rhogeessa and Tomopeas are allied tropical American types. Murina, with the subgenus Harpiocephalus, has i. 2/3, p. 2/2, and includes several small bats distinguished by the prominent tube-like nostrils and hairy interfemoral membrane. M. suilla, from Java, the Malay and neighbouring islands, is a well-known species, and the closely allied M. hilgendorfi is from Japan. The remaining species are from the Himalaya, Tibet and Ceylon; and apparently restricted to the hill-tracts of the countries in which they are found. Next to Vespertilio the genus Myotis (divisible into several subgenera), with i. 2/3, p. 3/3, includes the largest number of species, and has rather a wider geographical distribution in both hemispheres, one species being recorded from the Navigator Islands. The species may be recognized by the peculiar character of the pairs of upper incisors on each side, the cusps of which diverge from each other, by the large number of premolars, of which the second upper is always small, and by the oval elongated ear and narrow tragus. The British M. bechsteini and M. nattereri are examples of this group. Cerivoula (Kerivoula), which also has p. 3/3, is distinguished by the parallel upper incisors and the large second upper premolar. There are numerous African and Indo-Malayan species, of which C. picta, from India and Indo-Malay, is characterized by its brilliant orange fur, and membranes variegated with orange and black. The genus includes delicately formed insectivorous, tropical, forest-haunting bats, whose colouring approximates them to the ripe bananas among which they often pass the daytime.

Another subgroup, Minioptereae, is represented solely by the genus Miniopterus, with i. 2/3, p. 2/3. The incisors are separated from one another in front and from the canines; the first phalange of the middle finger is very short, the crown of the head elevated, and the tail long. The genus is represented by some half-dozen Old World species, among which the typical M. schreibersi ranges from Europe, southern Asia, and Africa to Japan and Australasia.





The last subfamily is that of the Molossinae, included by Dobson in the family Emballonuridae. In this group the premaxillae are in contact or but very slightly separated; the ears are large, with the tragus small; the dental formula is i. 1/1 (1/2 or 1/3), c. 1/1, p. 1/2 (2/2), m. 3/3; and the fibula is strongly developed. In their blunt muzzles and many other features these bats undoubtedly resemble the Emballonuridae, from the typical members of which they differ by the production of the thick tail far beyond the margin of the interfemoral membrane. They are further characterized by their broad and stout feet, in which the first, and in most cases also the fifth, toe is thicker than the rest, and furnished with long bent hairs; and by the presence of callosities at the base of the thumbs, and a single pair of large upper incisors occupying the centre of the space between the canines. The feet are free from the wing-membrane, which folds up under the fore-arm and legs; the interfemoral membrane is retractile, being movable backwards and forwards along the tail; this power of varying its superficial extent confers on these bats great dexterity in changing the direction of flight. All are able to walk or crawl well, and spend much of their time on trees. The genus Chiromeles, with i. 1/1, c. 1/1, p. 1/2, m. 3/3, the first hind-toe much larger than and separate from the others, and the widely sundered ears, is represented by C. torquata, a large bat of peculiar aspect, inhabiting the Indo-Malay countries. This species is nearly naked, a collar only of thinly spread hairs half surrounding the neck, and is remarkable for its enormous throat-sac and nursing-pouches. The former consists of a semicircular fold of skin forming a pouch round the neck beneath, concealing the orifices of subcutaneous pectoral glands which discharge an oily fluid of offensive smell. The nursing-pouch is formed on each side by an extension of a fold of skin from the side of the body to the inferior surfaces of the humerus and femur. In the anterior part of this pouch the teat is placed. The typical genus Molossus (fig. 21) includes the mastiff-bats, characterized by the dental formula i. 1/1 or 1/2, p. 1/2 or 2/2; and by the upper incisors being close together in front. The genus is restricted to the tropical and subtropical regions of the New World. M. obscurus, a small species common in tropical America, inhabits the hollow trunks of palms and other trees and the roofs of houses. The males and females live apart (as is the case in most if not all bats). In West Africa the mastiff-bats are represented by Eomops, with one species; while Nyctinomops includes a number of tropical American species more nearly related to the next genus, in which some of them (fig. 22) were formerly included. The widely spread Nyctinomus, with i. 1/3 or 1/2, p. 2/2 or 1/2, and the upper incisors separate in front, includes numerous species inhabiting the tropical and subtropical parts of both hemispheres. The lips of the bats of this genus are even more expansible than in Molossus, in many of the species (fig. 22) showing vertical wrinkles. N. toeniotis (or cestonii), one of the largest species, alone extends into Europe, as far north as Switzerland. N. johorensis, from the Malay Peninsula, is remarkable for the extraordinary form of its ears. N. brasiliensis is common in tropical America, and extends as far north as California.

Myzopoda and Mystacops.



Here may be conveniently noticed two very rare and aberrant bats, Myzopoda (or Myxopoda) aurita of Madagascar, and Mystacops (or Mystacina) tuberculatas of New Zealand, the latter of which is believed to be well-nigh, if not entirely, exterminated. Their systematic position and affinities are somewhat uncertain; but in the opinion of O. Thomas[2] the former should typify a separate family, Myzopodidae, in which the latter may also find a place. From all other bats Myzopoda is distinguished by the presence of a peculiar mushroom-shaped organ at the base of the large ear, and by the union of the tragus with the latter, on the inner base of which it forms a small projection. There are three phalanges in the middle finger; and the whole inferior surface of the thumb supports a large sessile horseshoe-shaped adhesive pad, with the circular margin directed forwards and notched along its edge, while a smaller pad occupies part of the sole of the hind-foot. Mr Thomas regards this bat as related on the one hand to the subfamily Mormopsinae of the Phyllostomatidae, and on the other to the Natalinae among the Vespertilionidae; both these groups being regarded by him as of family rank.

Mystacops resembles Myzopoda in having three phalanges to the middle finger, but differs in that the tail perforates the interfemoral membrane to appear on its upper surface in the manner characteristic of the Emballonuridae. The greater part of the wing-membrane is exceedingly thin, but a narrow portion along the fore-arm, the sides of the body, and the legs, is thick and leathery, and beneath this thickened portion the wings are folded. Other peculiarities of structure are found in the form of the claws of the thumbs and toes, each of which has a small heel projecting from its concave surface near the base, also in the sole of the foot and inferior surface of the leg, as shown in fig. 23. The plantar surface, including the toes, is covered with soft and very lax, deeply wrinkled skin, and each toe is marked by a central longitudinal groove with short grooves at right angles to it. The lax wrinkled integument is continued along the inferior flattened surface of the ankle and leg. These peculiarities appear to be related to climbing habits in the species.

Extinct Bats.

Palaeontology tells us nothing with regard to the origin of the Chiroptera, all the known fossil species, some of which date back to the Oligocene, being more or less closely allied to existing types, and therefore of comparatively little interest. The origin of the order from primitive insectivorous mammals must have taken place at least as early as the Lower Eocene. It is, however, noteworthy that several of the earlier extinct species appear to be related to the Rhinolophidae, which is the most generalized family of the order. Remains of Pteropodidae belonging to existing genera occur in the caves of tropical countries in the eastern hemisphere; and the skeleton of an extinct generic type, Archaeopteropus, has been obtained from the Miocene lignite of Italy, which indicates a form to a certain extent transitional in character between typical fruit-bats and the insectivorous bats. The tail, for instance, which in most modern fruit-bats is rudimentary, with only three or four vertebrae, in the fossil has eight complete vertebrae; while the teeth of the extinct form are distinctly cusped. Whether, however, the tail is longer than in the existing Notopteris of Fiji and New Guinea, or whether the molars are more distinctly cusped than is the case with the Solomon Island Pteropus (Pteralopex), is not stated. Still, the fact that the Miocene fruit-bat does show certain signs of approximation to the insectivorous (and more generalized) section of the order is of interest. Of the Oligocene forms, Pseudorhinolophus of Europe is apparently a member of the Rhinolophidae; but the affinities of Alastor and Vespertiliavus, which are likewise European, are more doubtful, although the latter may be related to Taphozous. The North American Vespertilio (Vesperugo) anemophilus and the European V. aquensis and V. parisiensis are, on the other hand, members of the Vespertilionidae, the last being apparently allied to the serotine (V. serotinus).

AUTHORITIES.—The above article is based to some extent on the article in the 9th edition of this work by G.E. Dobson, whose British Museum "Catalogue" is, however, now obsolete. Professor H. Winge's "Jordfundae og nulevende Flagermus (Chiroptera)," published in E. Mus. Lundi (Copenhagen, 1892), contains much valuable information; and for Pteropodidae Dr P. Matschie's Megachiroptera (Berlin, 1899), should be consulted. For the rest the student must refer to namerous papers by G.M. Allen, K. Andersen, F.A. Jentink, G.S. Miller, T.S. Palmer, A.G. Rehn, O. Thomas and others, in various English and American zoological serials, all of which are quoted in the volumes of the Zoological Record. (R. L.*)

FOOTNOTES:

[1] Bull. Amer. Mus. Nat. Hist. vol. xii. (1899).

[2] Proc. Zool. Soc. (London, 1904), vol. ii.

CHIRU, a graceful Tibetan antelope (Pantholops Hodgsoni), of which the bucks are armed with long, slender and heavily-ridged horns of an altogether peculiar type, while the does are hornless. Possibly this handsome antelope may be the original of the mythical unicorn, a single buck when seen in profile looking exactly as if it had but one long straight horn. Although far from uncommon, chiru are very wary, and consequently difficult to approach. They are generally found in small parties, although occasionally in herds. They inhabit the desolate plateau of Tibet, at elevations of between 13,000 and 18,000 ft., and, like all Tibetan animals, have a firm thick coat, formed in this instance of close woolly hair of a grey fawn-colour. The most peculiar feature about the chiru is, however, its swollen, puffy nose, which is probably connected with breathing a highly rarefied atmosphere. A second antelope inhabiting the same country as the chiru is the goa (Gazella picticaudata), a member of the gazelle group characterized by the peculiar form of the horns of the bucks and certain features of coloration, whereby it is markedly distinguished from all its kindred save one or two other central Asian species. The chiru, which belongs to the typical or antilopine section of antelopes, is probably allied to the saiga. (R. L.*)



CHIRURGEON, one whose profession it is to cure disease by operating with the hand. The word in its original form is now obsolete. It derives from the Mid. Eng. cirurgien or sirurgien, through the Fr. from the Gr. [Greek: cheirourgos], one who operates with the hand (from [Greek: cheir], hand, [Greek: ergon], work); from the early form is derived the modern word "surgeon." "Chirurgeon" is a 16th century reversion to the Greek origin. (See SURGERY.)



CHISEL (from the O. Fr. cisel, modern ciseau, Late Lat. cisellum, a cutting tool, from caedere, to cut), a sharp-edged tool for cutting metal, wood or stone. There are numerous varieties of chisels used in different trades; the carpenter's chisel is wooden-handled with a straight edge, transverse to the axis and bevelled on one side; stone masons' chisels are bevelled on both sides, and others have oblique, concave or convex edges. A chisel with a semicircular blade is called a "gouge." The tool is worked either by hand-pressure or by blows from a hammer or mallet. The "cold chisel" has a steel edge, highly tempered to cut unheated metal. (See TOOL.)



CHISLEHURST, an urban district in the Sevenoaks parliamentary division of Kent, England, 11-1/4 m. S.E. of London, by the South-Eastern & Chatham railway. Pop. (1901) 7429. It is situated 300 ft. above sea-level, on a common of furze and heather in the midst of picturesque country. The church of St Nicholas (Perpendicular with Early English portions, but much restored) has a tomb of the Walsingham family, who had a lease of the manor from Elizabeth; Sir Francis Walsingham, the statesman, being born here in 1536. Another statesman of the same age, Sir Nicholas Bacon, was born here in 1510. Near the church is an ancient cockpit. The mortuary chapel attached to the Roman Catholic church of St Mary was built to receive the body of Napoleon III., who died at Camden Place in 1873; and that of his son was brought hither in 1879. Both were afterwards removed to the memorial chapel at Farnborough in Hampshire. Camden Place was built by William Camden, the antiquary, in 1609, and in 1765 gave the title of Baron Camden to Lord Chancellor Pratt. The house was the residence not only of Napoleon III., but of the empress Eugenie and of the prince imperial, who is commemorated by a memorial cross on Chislehurst Common. The house and grounds are now occupied by a golf club. There are many villa residences in the neighbourhood of Chislehurst.



CHISWICK, an urban district in the Ealing parliamentary division of Middlesex, England, suburban to London, on the Thames, 71/2 m. W. by S. of St Paul's cathedral. Pop. (1901) 29,809. The locality is largely residential, but there are breweries, and the marine engineering works of Messrs Thornycroft on the river. Chiswick House, a seat of the duke of Devonshire, is surrounded by beautiful grounds; here died Fox (1806) and Canning (1827). The gardens near belonged till 1903 to the Royal Horticultural Society. The church of St Nicholas has ancient portions, and in the churchyard is the tomb of William Hogarth the painter, with commemorative lines by David Garrick. Hogarth's house is close at hand. Chiswick Hall, no longer extant, was formerly a country seat for the masters and sanatorium for the scholars of Westminster school. Here in 1811 the Chiswick Press was founded by Charles Whittingham the elder, an eminent printer (d. 1840).



CHITA, a town of east Siberia, capital of Transbaikalia, on the Siberian railway, 500 m. E. of Irkutsk, on the Chita river, half a mile above its confluence with the Ingoda. Pop. (1883) 12,600; (1897) 11,480. The Imperial Russian Geographical Society has a museum here. Several of the palace revolutionaries, known as Decembrists, were banished to this place from St Petersburg in consequence of the conspiracy of December 1825. The inhabitants support themselves by agriculture and by trade in furs, cattle, hides and tallow bought from the Buriats, and in manufactured wares imported from Russia and west Siberia.



CHITALDRUG, a district and town in the native state of Mysore, India. The district has an area of 4022 sq. m. and a population (1901) of 498,795. It is distinguished by its low rainfall and arid soil. It lies within the valley of the Vedavati or Hagari river, mostly dry in the hot season. Several parallel chains of hills, reaching an extreme height of 3800 ft., cross the district; otherwise it is a plain. The chief crops are cotton and flax; the chief manufactures are blankets and cotton cloth. The west of the district is served by the Southern Mahratta railway. The largest town in the district is Davangere (pop. 10,402). The town of CHITALDRUG, which is the district headquarters (pop. 1901, 5792), was formerly a military cantonment, but this was abandoned on account of its unhealthiness. It has massive fortifications erected under Hyder Ali and Tippoo Sahib towards the close of the 18th century; and near it on the west are remains of a city of the 2nd century A.D.



CHITON, the name[1] given to fairly common littoral animals of rather small size which belong to the phylum Mollusca, and, in the possession of a radula in the buccal cavity, resemble more especially the Gastropoda. Their most important characteristic in comparison with the latter is that they are, both in external and internal structure, bilaterally symmetrical. The dorsal integument or mantle bears, not a simple shell, but eight calcareous plates in longitudinal series articulating with each other. The ventral surface forms a flat creeping "foot," and between mantle and foot is a pallial groove in which there is on each side a series of gills. Originally the Chitons were placed with the limpets, Patella, in Cuvier's Cyclobranchia, an order of the Gastropoda. In 1876 H. von Jhering demonstrated the affinities of Neomenia and Chaetoderma, vermiform animals destitute of shell, with the Chitons, and placed them all in a division of worms which he named Amphineura. The discovery by A.A.W. Hubrecht in 1881 of a typical molluscan radula and odontophore in a new genus Proneomenia, allied to Neomenia, showed that the whole group belonged to the Mollusca. E. Ray Lankester (Ency. Brit., 9th ed., 1883) placed them under the name Isopleura as a subclass of Gastropoda. Paul Pelseneer (1906) raised the group to the rank of a class of Mollusca, under von Jhering's name Amphineura.

The Amphineura are divided into two orders: (1) the Polyplacophora, or Chitons; (2) the Aplacophora, or forms without shells, Neomenia, Chaetoderma and their allies.

Order I.—POLYPLACOPHORA

[Illustration: FIG. 1.—Three views of Chiton.

A. Dorsal view of Chiton Wosnessenksii, Midd., showing the eight shells. (After Middendorf.)

B. View from the pedal surface of a species of Chiton from the Indian Ocean, p, foot; o, mouth (at the other end of the foot is seen the anus raised on a papilla); kr, oral fringe; br, the numerous ctenidia (branchial plumes); spreading beyond these, and all round the animal, is the mantle-skirt. (After Cuvier.)

C. The same species of Chiton, with the shells removed and the dorsal integument reflected, b, buccal mass; m, retractor muscles of the buccal mass; ov, ovary; od, oviduct; i, coils of intestines; ao, aorta; c', left auricle; c, ventricle.]



Each of the eight valves of the shell is made up of two distinct calcareous layers: (a) an outer or upper called the tegmentum, which is visible externally; (b) a deeper layer called articulamentum which is porcellaneous, quite compact, and entirely covered by the tegmentum. In the lower forms the two layers are coextensive and have smooth edges, but in the higher forms the articulamentum projects laterally beyond and beneath the tegmentum into the substance of the mantle. These projections are termed insertion plates; they are usually slit or notched to form teeth, the edges of which may be smooth and sharp, or may be crenulated. The anterior margin of each valve except the first is provided with two projections called sutural laminae which underlie the posterior margin of the preceding valve.



The tegmentum is formed by the fold of mantle covering the edge of the articulamentum, and extends over the latter from the sides. It is the first part of the shell formed in development. The tegmentum is much reduced in Acanthochiton, and absent in the adult Cryptochiton. The tegmentum is pierced by numerous vertical ramified canals which contain epithelial papillae of the epidermis. These papillae form pallial sense-organs, containing nerve-end bulbs, covered by a dome of cuticle, and innervated from the pallial nerve-cords. They are termed according to their size, micraesthetes and megalaesthetes. In the common species of Chiton and many others of the family Chitonidae the megalaesthetes are developed into definite eyes, the most complicated of which have retina, pigment within the eye, cornea and crystalline lens (intra-pigmental eyes) (fig. 2). The eyes are arranged in rows running diagonally from the median anterior beak of each valve to its lateral borders There may be only one such row on either side, or many rows. In some species the total number present amounts to thousands.



A. Neomenia and Proneomenia. B. Chaetoderma. C. Chiton. o, Mouth. a, Anus. d, Alimentary canal. l, Liver (digestive gland).]

Branchiae.—The series of gills may extend the whole length of the body in the pallial groove, or may be confined to the posterior end. Each gill has the structure of a typical molluscan ctenidium, consisting of an axis bearing an anterior and posterior row of filaments or lamellae. The gills are thus metamerically repeated; there may be from four to eighty pairs, but there is often a numerical asymmetry on the two sides. The largest pair of branchiae is placed immediately behind the renal openings and corresponds to the single pair of other molluscs, the organs being repeated anteriorly only (Metamacrobranchs) or anteriorly and posteriorly (Mesomacrobranchs).

Intestine.—The digestive tube in the Polyplacophora, which are herbivorous, is longer than the body, and thrown into a few coils, the anus being median and posterior. The mouth leads into the buccal cavity, on the ventral side of which opens the radular caecum. Each transverse row of teeth of the radula contains 17 teeth, one of which is median, while the second and the fifth on each side are enlarged. Two pairs of glands open into the buccal cavity, and at the junction of pharynx and oesophagus is another pair called the sugar glands. The stomach is surrounded by the liver or digestive gland, consisting of two lobes which are symmetrical in the young animals, but in the adult the right lobe is anterior and smaller.



Coelom, Gonads and Excretory Organs.—As in other molluscs the coelom is represented by a large pericardial cavity, situated above the intestine posteriorly, and a generative sac which is single and median and situated in front of the pericardium, except in the Nuttalochiton hyadesi, where the gonads are in a similar position, but are paired. The excretory organs are coelomoducts with an internal ciliated opening into the pericardium and an opening to the exterior. Both the openings are close together, the external opening being just in front of the principal gill near the posterior end of the body. The renal tube is doubled on itself, its middle part where the bend occurs being situated more or less anteriorly. The excretory surface is increased by numerous ramified caeca which extend beneath the body wall laterally and ventrally, and open into the tube (fig. 6). The sexes are distinct, and the ovary is frequently greenish in colour, the testis red. The gonad is transversely wrinkled and lies between the aorta and the intestine, extending from the pericardium to the anterior end of the body. A simple gonaduct on each side arises from the gonad near its posterior end and passes first forwards, then backwards, and lastly outwards to the external opening in the pallial groove, anterior to the renal aperture. There may be from one to nine gills between the genital and renal pores.

Heart and Vascular System.—The heart is enclosed in the pericardium, and consists of a median elongated ventricle and a pair of lateral auricles, so that the structure somewhat resembles that in the Lamellibranchiata. The openings of the auricles into the ventricle vary in different forms. In many of the lower forms (Lepidopleuridae, Mopalidae, Ischnochitonidae) the opening on each side is single and anterior. In the true Chitonidae there are generally two apertures on each side, and in two species three or four, another instance of the tendency to metameric repetition in the group. The auricles are connected with one another posteriorly behind the ventricle. The ventricle leads into a single anterior median aorta. As in other molluscs, the arteries do not extend far, but lead into inter-visceral blood-spaces. The venous blood is conducted from the tissues to a large sinus on either side above the pallial groove, and from this sinus passes to the gills by an afferent vessel in each gill on the internal or pedal margin of the axis. The oxygenated blood is carried from each gill by an efferent vessel on the external or pallial side of the axis to another longitudinal vessel which leads to the auricle on each side.



Nervous System.—There are no well-marked specialized ganglia in the central nervous system, nerve-cells being distributed uniformly along the cords. There are two pairs of longitudinal cords, a pedal pair situated ventrally and united beneath the intestine by numerous commissures, and a pallial pair situated laterally and continuous with one another above the rectum (fig. 7). The four cords are all connected anteriorly with the cerebral commissure which lies above the buccal mass anteriorly. From the points where the cords meet the cerebral commissure, arise on each an anterior labial commissure and a stomatogastric commissure. The letter bears two ganglion swellings, the buccal ganglia. The labial commissure gives off a subradular commissure which also bears two ganglia, these being in close relation to a special sense-organ called the subradular organ, an epithelial projection with nerve-endings, lying in front of the radula and probably gustatory in function. One osphradium or branchial olfactory organ is usually present on each side, on either side of the anus on the inner wall of the mantle, near the base of the last gill. In Lepidopleuridae an osphradium occurs at the base of each gill. The sense organs of the shell-valves have already been described.

Development.—The eggs may be laid separately invested by a chitinous envelope, or as in Ischnochiton magdalenensis they may form strings containing nearly 200,000 eggs, or the ova may be retained in the pallial groove and undergo development there, as in Chiton polii and Hemiarthrum setulosum. One species Callistochiton viviparus is viviparous and its ova develop without a larval stage in the maternal oviduct. Segmentation is total and at first regular, and is followed by invagination, the blastopore passing to the position of the future mouth. By the development of a ciliated ring just in front of the mouth the embryo becomes a trochosphere. In the centre of the praeoral lobe is a tuft of cilia. Just behind the ciliated ring is a pair of larval eyes which disappear in the adult; these correspond to the cephalic eyes of Lamellibranchs. An ectodemic invagination forms a large mucous gland on the foot, which is more or less atrophied in adult life. The gonads originate by proliferation of the anterior wall of the pericardium. The shell-valves arise as transverse thickenings of the dorsal cuticle behind the ciliated ring, the tegmentum being the first part formed.

Classification.





Suborder I. EOPLACOPHORA, Pilsbry.—Tegmentum coextensive with articulamentum, or the latter projecting in smooth unslit plates.

Fam. 1. Lepidopleuridae.—Terminal margins of end valves never elevated; form oval or oblong. Lepidopleurus cancellatus, Sow. North Atlantic and Mediterranean; various abyssal species. Hanleya hanleyi, Bean, north Atlantic. Hemiarthrum Microplax. The extinct Gryptochitonidae, Pilsbry, with other Palaeozoic genera, narrow and elongated in form with terminal margins of end valves elevated, belong to this group.

Suborder II. MESOPLACOPHORA, Pilsbry.—Insertion plates well developed and slit.

Fam. 2. Ischnochitonidae.—All the valves with slits, and the inner layer well covered by the outer.

Subfam. 1. Ischnochitoninae.—No shell-eyes: sutural laminae separated; slits in the valves 1-7 do not correspond with the ribs of the tegmentum. Ischnochiton, Trachydermon, Chaetopleura, Stenoplax, Stenoradsia.

Subfam. 2. Callochitoninae. With shell-eyes and united sutural laminae. Callochiton laevis, North Atlantic and Mediterranean.

Subfam. 3. Callistoplacinae. No shell-eyes, slits in the valves 1-7 corresponding with the ribs of the tegmentum. Callistochiton (viviparous). Nuttalochiton.

Fam. 3. Mopaliidae. Each intermediate valve with a single slit; girdle hairy. Mopalia, Placiphorella, Plaxiphora, Placophoropsis.

Fam. 4. Acanthochitonidae. Valves immersed in the girdle, with small tegmentum. Acanthochiton (A. fascicularis, North Atlantic and Mediterranean). Spongiochiton, Katharina, Amicula, Cryptochiton (C. stelleri, arctic).

Fam. 5. Cryptoplacidae. Vermiform, with thick girdle and small valves; insertion and sutural plates strongly drawn forward, sharp and smooth. Cryptoplax, Choneplax.

Suborder III. TELEOPLACOPHORA, Pilsbry.—All the valves, or at least the seven anterior, with insertion plates cut into teeth by slits.

Fam. 6. Chitonidae. Characters of the suborder.

Subfam. 1. Chitoninae. No extra-pigmental eyes; insertion plates with pectinations between the fissures. Chiton, Eudoxochiton, Trachyodon, Radsia.

Subfam. 2. Toniciinae. Extra-pigmental shell-eyes. Tonicia, Acanthopleura, Enoplochiton, Onithochiton, Schizochiton, Lorica, Loricella, Liolophura.

Order 2.—APLACOPHORA, von Jhering.

Chaetoderma was first described by S. Loven, in 1841, and was for a long time believed to be a Gephyrean worm. Neomenia, mentioned first by Michael Sars in 1868 under the name Solenopus, was afterwards included among the Opisthobranchs by J. Koren and D.C. Danielssen. C. Gegenbaur placed the two genera in a division of Vermes which he called Solenogastres.

The chief points in which the Aplacophora differ from the Polyplacophora are: (1) they are worm-like in shape; (2) there is no distinct foot, and the mantle bears no shell-valves, but only numerous calcareous spicules; (3) the digestive tube is straight.

Neomenia and its allies are marine animals living at depths of 15 to 800 fathoms on soft muddy ground; they are found crawling on corals and hydrozoa, on which they feed. The British genera are: Neomenia, Rhopalomenia and Myzomenia. They have been taken in nearly all seas except the South Atlantic and S.E. and N.W. Pacific. About forty species are known. Chaetoderma, of which nine species have been described, has similar habits and distribution, but feeds chiefly on Protozoa. The order Aplacophora is divided into two suborders.

Suborder I. NEOMENIOMORPHA.—Aplacophora with a distinct longitudinal ventral groove; bisexual with paired genital glands and no distinct liver. The whole of the skin except the ventral groove corresponds to the mantle of Chiton. The cuticle, in some species very thick, contains numerous spicules which are long, hollow and calcified; they are secreted by epithelial papillae. In some species there are also sensory papillae comparable to the aesthetes of Chitons. A small longitudinal projection in the ventral groove represents the foot. Into the groove open mucous glands, a large one anteriorly and another opening into a posteriorly cloacal, branchial cavity.



Branchiae.—In Neomeniidae and most of the Parameniidae there is a circlet of gills on the inner walls of the cloacal chamber. These gills are simple folds or laminae of the body wall. In other species they are absent.

Intestine.—-The mouth opens into a muscular pharynx lined by a thick cuticle. Into the pharyngeal cavity open salivary glands and radular sac. The former are paired and ventral, and open on a subradular prominence. In some species there is a second dorsal pair. Neomenia and other genera have no salivary glands.

The radula when present comprises several transverse rows of teeth, and each transverse row may have several teeth (polystichous), two teeth (distichous), or one tooth (monostichous). It is a curious fact that in the original type Neomenia the radula is entirely absent, as it likewise is in several genera of Proneomeniidae. The oesophagus is short and leads into a long, straight stomach, provided with numerous symmetrical lateral caeca. The stomach opens into a short straight rectum which opens into the branchial chamber.

Coelom, Gonads and Excretory Organs.—The coelom differs from that of the Chitons in the fact that the cavities of the genital organs are continuous with it, and in the fact that there is only one pair of coelomoducts resembling the renal organs of Chitons, but serving also as genital ducts. The gonads are paired and hermaphrodite, they form a pair of anterior prolongations of the pericardium, extending nearly to the anterior end of the body. Ova are developed on the median, spermatozoa on the outer wall of each genital tube. The pericardium is ciliated internally on its dorsal and lateral walls. The urino-genital tubes arise from the posterior angles of the pericardium, pass first forwards, then backwards, and unite to open by a common opening into the cloaca below the anus except in Strophomenia, where the openings are separate. Usually each tube is provided with caecal appendages on its proximal portion, and these serve as vesiculae seminales, while the distal portion is enlarged and glandular and secretes the egg-shell.

Heart and Vascular System.—There is a heart in the pericardium consisting of a median ventricle attached, except in Neomenia, to the dorsal wall of the pericardium, and in Neomenia a pair of auricular ducts returning blood from the gills to the ventricle. The aorta is not independent as in Chitons, but is a sinus like the other channels of the circulation. A single median ventral sinus passes backwards to the gills or cloaca. The blood is coloured red by haemoglobin in blood corpuscles.

Nervous System.—Ganglionic enlargements are more conspicuous than in the Chitons. In front of the buccal mass is a median cerebral ganglion. From this pass off two pairs of cords, the pleural and pedal, in Proneomenia separate from their origin, in Neomenia united at first and diverging at a pleural ganglion. The pedal cords anteriorly form a pair of pedal ganglia united by a thick commissure. The supra-rectal commissure may be present and bear an ovoid ganglion; or may be wanting. With regard to sense organs the epithelial papillae of the mantle have been mentioned. There is also in some genera a median retractile sensory papilla on the dorsal posterior surface above the rectum, not covered by the cuticle.

Development has only been described in Myzomenia banyulensis, by G. Pruvot. It closely resembles in the early stages that of Chitons. The external surface of the trochosphere is formed of a number of ciliated test-cells. The ectoderm behind the ciliated ring develops spicules, and the post-oral region of the larva elongates. Later the ciliated ring or velum disappears and seven imbricated calcareous plates, made up of flattened spicules, are formed on the dorsal surface. This appears to indicate that the Neomeniomorpha are descended from Chiton-like ancestors, and that they have lost their shell valves.

Classification of the NEOMENIOMORPHA.—Fam. 1. Lepidomeniidae. Slender, tapering behind, with subventral cloacal orifice; thin cuticle without papillae; flattened spicules; no gills. Lepidomenia, Ismenia, Ichthyodes, Stylomenia, Dondersia, Nematomenia, Myzomenia, M. banyulensis, Mediterranean and Plymouth.

Fam. 2. Neomeniidae. Short, truncate in front and behind; cloacal orifice transverse; gills present; rather thin cuticle; no radula. Neomenia (N. carinata, N. Atlantic and N. and N.W. Scotland), Hemimenia.

Fam. 3. Proneomeniidae. Elongated, cylindrical, rounded at both ends; thick cuticle with acicular spicules; radula polystichous or wanting. Proneomenia, Amphimenia, Echinomenia, Rhopalomenia (R. aglaopheniae, Mediterranean and Plymouth), Notomenia, Pruvotia, Strophomenia.

Fam. 4. Parameniidae. Short and truncated in front; thick cuticle, often without papillae; gills and radula present. Paramenia, Macellomenia, Pararhopalia, Dinomenia, Cyclomenia, Proparamenia, Uncimenia, Kruppomenia.

Suborder II. CHAETODERMOMORPHA.—Aplacophora without distinct ventral groove, with single median unisexual gonad, with differentiated hepatic sac, and with cloacal chamber furnished with two bipectinate gills. There are only two genera in this suborder: Chaetoderma, and Limifossor from Alaska. The characters therefore are very uniform. The body is worm-like and cylindrical, the posterior half a little thicker than the anterior; the posterior extremity forms the enlarged funnel-like branchial or cloacal chamber. The anterior extremity is also somewhat enlarged. The whole surface is uniformly covered with short compressed calcareous spicula embedded in the cuticle.



Branchiae.—The single pair of branchiae are placed symmetrically right and left of the anus, and each has the structure of a ctenidium bearing a row of lamellae on each side as in the Polyplacophora.

Intestine.—The mouth is anterior, terminal and crescentic, and beneath it is a rounded ventral shield. On the floor of the pharynx or buccal mass is a rudimentary radula, which in many species consists of a single large tooth, bearing two small teeth or a row of teeth. In other species the radula is more of the usual type consisting of several transverse rows of two or three teeth each. Two pairs of salivary glands open into the buccal cavity. The digestive tube is straight and simple, wider in its anterior part, into which opens the duct of the hepatic caecum (fig. 4, B). The latter extends backwards on the ventral side of the intestine.

Coelom, Gonads and Excretory Organs.—These are closely similar in their relations to those of the Neomeniomorpha. The chief difference is that the gonad or generative portion of the coelom is single and median, opening into the pericardium by a single posterior aperture. The excretory organs or coelomoducts arise from the posterior corners of the pericardium, run forwards and then backwards to open by separate apertures lateral to the gills (fig. 5, A). There are no accessory generative organs.

The heart and vascular system are similar to those of the Neomeniomorpha, the only important differences being that the ventricle is nearly free in the pericardial cavity, and that the latter is traversed by the retractor muscles of the gills.

Nervous System.—There are two closely connected cerebral ganglia, from which arise the usual two pairs of nerve cords. Pallial and pedal on each side are closer together than in the other groups, and posteriorly they unite into a supra-rectal cord provided with a median ganglionic enlargement (fig. 7, C). A small stomatogastric commissure bearing two small ganglia arises from the cerebral ganglia and surrounds the oesophagus.

The development is at present entirely unknown.

General Remarks on the Amphineura.

The most important theoretical question concerning the Amphineura is how far do they represent the original condition of the ancestral mollusc? That is to say, we have to inquire which of their structural features is primitive and which modified. Their bilateral symmetry is obviously to be regarded as primitive, and the nervous system shows an original condition from which that of the asymmetrical twisted Gastropods can be derived. But in many other features both external and internal the three principal divisions differ so much from one another that we have to consider in the case of each organ-system which condition is the more primitive. According to Paul Pelseneer the Polyplacophora are the most archaic, the Aplacophora being specialized in (1) the great reduction of the foot, (2) the disappearance of the shell (Cryploplax among the Polyplacophora showing both reductions in progress), (3) the disappearance of the radula. But it is a widely recognized principle of morphology that a much modified animal is by no means modified to the same degree in all its organs. A form which is primitive on the whole may show a more advanced stage of evolution in some particular system of organs than another animal which is on the whole more highly developed and specialized. Thus the independent metamerism of certain organs in the Chitons is not primitive but acquired within the group: e.g. the shell valves and the ctenidia. And although embryology seems to prove that the Neomeniomorphs are derived from forms with a series of shell-valves, nevertheless it seems probable that the calcareous spicules which alone are present in adult Aplacophora preceded the solid shell in evolution.

It is held by some morphologists that the mollusc body is unsegmented, and therefore is to be compared to a single segment of a Chaetopod or Arthropod. In this case there should be only one pair of coelomoducts in the adult, the pair of true nephridia which should also occur being represented by the larval nephridia. There should also be only a single coelom, or a pair of lateral coelomic cavities. On this view then the Aplacophora are more primitive than the Polyplacophora in the relations of coelom, gonad and coelomoducts; and the genital ducts of the Chitons have arisen either by metameric repetition within the group, or by the gradual loss of an original connexion between the generative sac and the renal tube, as in Lamellibranchs and Gastropods, the generative sac acquiring a separate duct and opening to the exterior on each side.

LITERATURE.—A. Sedgwick, "On certain Points in the Anatomy of Chiton," Proc. R. Soc. Lond. xxxiii., 1881; J. Blumrich, "Das Integument der Chitonen," Zeitsch. f. wiss. Zool. lii., 1891; A.C. Haddon, "Report on the Polyplacophora," Challenger Reports. Zool. pt. xliii., 1886; H.N. Moseley, "On the presence of Eyes in the Shells of certain Chitonidae, and on the structure of these Organs," Quart. Journ. Mic. Sci. new ser. xxv., 1885; A.A.W. Hubrecht, "Proneomenia Sluiteri," Nied. Arch. f. Zool. Suppl. 1., 1881; A. Kowalewsky and A.F. Marion, "Contr. a l'histoire des Solenogastres ou Aplacophores," Ann. Mus. Marseille, Zool. iii., 1887; A. Kowalewsky, "Sur le genre Chaetoderma," Arch. de zool. exper. (3) ix., 1901; P. Pelseneer, "Mollusca," Treatise on Zoology, edited by E. Ray Lankester, pt. v., 1906; E. Ray Lankester, "Mollusca," in the 9th ed. of this Encyclopaedia, to which this article is much indebted. (J. T. C.)

FOOTNOTE:

[1] The Gr. [Greek: chiton] was a garment in the shape of a loose tunic, varying at different periods: see COSTUME: Greek.

THE END