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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 See also:

GROUP

group belonged to the Mollusca. E.

See also:

Ray Lankester (Ency. Brit., 9th ed., 1883) placed them under the name Isopleura as a subclass of Gastropoda. See also:

Paul Pelseneer (19(36) raised the group to the See also:

RANK (O.Fr. rant or rent, mod. rang, generally connected with the O.E. and O.H.G. bring, a ring)

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 See also:

ALLIES, THOMAS WILLIAM (1813-1903)

allies. Order I.—POLYPLACOPHORA Each of the eight valves of the shell is made up of two distinct calcareous layers: (a) an See also:

OUTER

outer or upper called the tegmentum, which is visible externally; (b) a deeper layer called articula- :1rr m di` .od C A. Dorsal view of Chiton "Was- these, and all See also:

ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round the nessenksii, Midd., showing See also:

animal, is the mantle-skirt. the eight shells. (After (After Cuvier.) Middendorf.) C. The same See also:

SPECIES

species of Chiton, B. View from the pedal surface with the shells removed and of a species of Chiton from the dorsal integument re- the See also:

INDIAN

Indian Ocean. p, foot; fleeted. b, buccal See also:

mass; in, o, mouth (at the other end retractor muscles of the of the foot is seen the anus buccal mass; ov, ovary; raised on a papilla); kr, od, oviduct; i, coils of in- oral fringe; br, the numer- testines; ao, aorta; c', See also:

LEFT

left ous ctenidia (branchial See also:

AURICLE (from Lat. diminutive of auris, ear)

auricle; c, ventricle. plumes) ; spreading beyond mentum which is porcellaneous, quite compact, and entirely covered by the tegmentum. In the See also:

LOWER

lower forms the two layers are coextensive and have smooth edges, but in the higher forms (Moseley). 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 See also:

FORM (Lat. forma)

form See also:

TEETH (O.E. Eel); plural of tooth, O.E. top)

teeth, the edges of which may be smooth and See also:

sharp, or may be crenulated. The anterior margin of each See also:

VALVE (Lat. valva, a leaf of a double or folding door, allied to volvere, to roll, as of a door on its hinges)

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 See also:

FOLD

fold of mantle covering the A B C From Lankester, See also:

TREATISE

Treatise on See also:

ZOOLOGY (from Gr. Nov, a living thing, and ?byos, theory)

Zoology. position of gills. A. Lepidopleurus benthus. mouth; pa, mantle; pa', B. Boreochiton cinereus. anal See also:

LOBE

lobe of mantle; ps, C. Schizochiton incisus. a, pallial slit; te, pallial anus; f, foot; g, gills; m, tentacles. edge of the articulamentum, and extends over the latter from the sides. It is the first See also:

PART

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 See also:

VERTICAL (from Lat. vertex, highest point)

vertical ramified canals which contain See also:

EPITHELIAL, ENDOTHELIAL

epithelial papillae of the epidermis.

These papillae form pallial sense-See also:

ORGANS

organs, t–containing See also:

NERVE (Lat. nervus, Gr. vevpov, a bowstring)

nerve-end bulbs, covered by a See also:

DOME (Lat domus, house; Ital. duomo, cathedral)

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 See also:

FAMILY

family Chitonidae the megalaesthetes are See also:

DEVELOPED

developed into definite eyes, the most complicated of which have retina, pigment within the See also:

eye, cornea and crystalline See also:

lens (See also:

INTRA

intra-pigmental eyes) (fig. 2). The eyes are arranged in rows See also:

RUNNING

running diagonally from the median anterior See also:

BEAK (early forms beke and becke, from Fr. bec, late Lat. beccus, supposed to be a Gaulish word; the Celtic bec and beq, however, are taken from the English)

beak of each valve to its lateral See also:

BORDERS, THE

borders There may be only one such See also:

ROW, JOHN (c. 1525—1580)

row on either side, or many rows. In some species the See also:

TOTAL

total number See also:

PRESENT

present amounts to thousands. Branchiae.—The series of gills may extend the whole length of the See also:

BODY

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 See also:

AXIS (Lat. for " axle ")

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 ip 0 A. Neomenia and Proneomenia. B. Chaetoderma.

C. Chiton. o, Mouth. a, Anus. d, Alimentary See also:

canal. 1, See also:

LIVER (O. Eng. lifer; cf. cognate forms, Dutch lever, Ger. Leber, Swed. lefver, &c.; the O. H. Ger. forms are libara, lipora, &c.; the Teut. word has been connected with Gr. i'prap and Lat. jecur)

Liver (See also:

DIGESTIVE

digestive gland). A, B, C, D, 0, P, N, u, often a numerical 2symmetry 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). See also:

INTESTINE (Lat. intestinus, internal, usually in neuter plural intestina. from intus, within)

Intestine.—The digestive See also:

TUBE (Lat. tuba)

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 See also:

OESOPHAGUS (Gr. o'lvca=I will carry, and r/)ayeiv, to eat)

oesophagus is another pair called the See also:

SUGAR

sugar glands.

The See also:

STOMACH (Gr. vrbsaxos from arbga, a mouth)

stomach is surrounded by the liver or digestive Br -g Chaetoderma. Neornenia. Proneomenia. Chiton. Ovary. Pericardium. Nephridium. External See also:

APERTURE (from Lat. aperire, to open)

aperture ridium. gland, consisting of two lobes which are symmetrical in the See also:

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 See also:

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 See also:

close together, the external opening being just in front of the See also:

PRINCIPAL

principal gill near the posterior end of the body.

The renal tube is doubled on itself, its See also:

MIDDLE

middle part where the See also:

BEND

bend occurs being situated more or less anteriorly. The excretory surface is increased by numerous ramified caeca which extend beneath the body See also:

wall laterally and ventrally, and open into the tube (fig. 6). The sexes are distinct, and the ovary is frequently greenish in See also:

COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

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 249 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 thegenital and renal pores. See also:

Heart and Vascular See also:

SYSTEM

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 See also:

ARTERIES (Gr. ap-rnpia, probably from a"spew, to raise, but popularly connected by the ancients with ai7P, air)

arteries do not extend far, but See also:

lead into inter-visceral See also:

BLOOD

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 See also:

VESSEL (O. Fr. vaissel, from a rare Lat. vascellum, dim. of vas, vase, urn)

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. See also:

NERVOUS

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 See also:

UNITED

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 See also:

LETTER (through Fr. lettre from Lat. littera or litera, letter of the alphabet; the origin of the Latin word is obscure; it has probably no connexion with the root of linere, to smear, i.e. with wax, for an inscription with a stilus)

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 See also:

SPECIAL

special sense-See also:

ORGAN

organ called the subradular organ, an epithelial See also:

pro- jection with nerve-endings, nklying in front of the radula and probably gustatory in See also:

FUNCTION

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 See also:

BASE

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 in-vested by a chitinous envelope, or as in Ischnochiton magdatenensis 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 See also:

STAGE (Fr. 6/age; from Lat. stare, to stand)

stage in the maternal oviduct. Segmentation is total and at first See also:

REGULAR

regular, and is followed by invagination, the blastopore passing to the position of the future mouth. By the development of a ciliated See also:

RING (O.E. hring; a word common to Teutonic languages; and probably cognate with the Lat. circus, Gr. KtpKOS or KpLKOS, Skt'. chakra, wheel, circle, cf. also " harangue "); in art, a band of circular shape of varying sizes, made of any material and used f

ring lust in front of the mouth the embryo becomes a trochosphere. In the centre of the praeoral Iobe is a tuft of See also:

CILIA (plural of Lat. cilium, eyelash)

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 See also:

LIFE

life. The gonads originate by proliferation of the anterior wall of the pericardium. .The shell.

A S C External aperture of the genital duct of Chiton. Rectum. Cloacal or pallial chamber of Neomeniae and Chaetoderma. Ctenidia (branchial plumes). g, r, Cl, Br, of neph- After See also:

Halle: (Arbeiten toot. Instil.), See also:

Vienna, 1882. L, Edge of the mantle not removed in the front part of the specimen. s.o., Oesophagus. af, Anus. gg, Genital duct. go, External opening of the same. See also:

STEM (O. Eng. staefn, stemn, cf. Du. stain, Ger. Stamm, &c., probably related to " staff ")

Stem of the nephridium leading to no, its external aperture. Reflected portion of the nephridial stem.

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