Open main menu
This page has been proofread, but needs to be validated.
250
CHITON

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.

EB1911 Chiton Fig. 7.jpg EB1911 Chiton Fig. 8.jpg

After Hubrecht, loc. cit.

Fig. 7.—Diagrams of the nervous system of Amphineura.

A, Proneomenia.
B, Neomenia.
C, Chaetoderma.
D, Chiton.
c, Cerebral ganglia.
s, Sublingual ganglia.
v, Pedal (ventral) nerve-cord.
l, Visceral (lateral) nerve-cord.
pc. Post-anal junction of the visceral nerve-cords.

From Gegenbaur, Elements of Comp. Anatomy.

Fig. 8.—Anterior part of the nervous system
of Chiton cinereus, in more detail.

B, Buccal ganglia (concerned
with the odontophore).
C, Cerebral nerve-mass.
P, Pedal ganglion and commencement
of pedal nerve-cord.
pl, Visceral nerve-cord. The
sublingual ganglia are not lettered.

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. Lovén, 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.

EB1911 Chiton Fig. 9.jpg
Fig. 9.Neomenia carinata, Tullberg (after Tullberg).
A, Lateral view.
B, Ventral view.
C, Dorsal view.
D, Ventral view of a more
  extended specimen.
a, Anterior.
b, Posterior extremity.
c, Furrow, in which the narrow
  foot is concealed.

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