Chapter XIV: Class Gasteropoda--Amphineura and Prosobranchiata

=Order I. Amphineura=

Bilaterally symmetrical Mollusca, anus at the terminal end of the body, dorsal tegument more or less furnished with spicules.

=Sub-order 1.= =Polyplacophora= (Chitons).--Foot co-extensive with ventral surface of the body, dorsum with eight transverse plates, articulated (except in _Chitonellus_), a row of ctenidia on each side between the mantle and the foot. Silurian ----.

The Chitons are found in all parts of the world, ranging in size from a length of about half an inch to six inches or more in the giant _Cryptochiton_. Although in the main sub-littoral, they occur at very great depths; the _Challenger_ dredged _Leptochiton benthus_ Hadd. at 2300 fathoms. _Chiton Polii_ exceptionally occurs at Malta--_teste_ MacAndrew--above sea margin, but within reach of the ripple. As a rule, the Chitons live in concealment, on the under surface of stones or in deep and narrow fissures in the rocks. When the stone to which they are attached is turned over, they crawl slowly to the side which is not exposed, as if disliking the light. An undescribed species, however, which I took at Panama, crawled quite as fast as an ordinary snail. _Chiton fulvus_ Wood, apparently is accustomed to crawl with some rapidity. MacAndrew took it in abundance on his anchor chain in Vigo Bay every time his yacht was got under weigh. He also found it crawling in sand on the shore, to which habit is no doubt due its extreme cleanness and freedom from the foreign growths which are so characteristic of many of the species. When detached a _Chiton_ contracts the muscles of the whole body, and rolls up into a ball like a wood-louse.

The _Polyplacophora_ are characterised, _externally_, by their usually articulated shell of eight _plates_ or _valves_, which is surrounded and partly kept in position by a muscular _girdle_. These plates overlap like tiles on a roof in such a way that the posterior edge of the first, cephalic, or anterior valve projects over the anterior edge of the succeeding valve, which in its turn overlaps the next, and so on throughout. Seven-valved monstrosities very rarely occur.

A certain portion of each valve is covered either by the girdle or by the valve next anterior to it. This portion, which is whitish in colour and non-porous in structure, forms part of an inner layer which underlies the rest of the substance of the valve, and is called the _articulamentum_. The external portion of the valves, or _tegmentum_, is generally more or less sculptured, and is largely composed of chitin, impregnated with salts of lime, thus answering more to a cuticle than to a shell proper. It is very porous, being pierced by a quantity of minute holes of two sizes, known as _megalopores_ and _micropores_, which are connected together by minute canals containing what is probably fibrous or nerve tissue, the mouths of the pores being occupied by sense organs connected with these nerves. The tegmentum of the six intermediate valves is generally divided into three triangular areas by two more or less prominent ribs, which diverge from the neighbourhood of the median _beak_ or _umbo_. The space enclosed between these ribs is known as the _median area_ or _jugum_, the other two spaces as the _lateral areas_ or _pleura_. The ribs terminate with the edge of the tegmentum, and are not found on the articulamentum. In certain genera these areas are either non-existent, or are not distinctly marked. The sculpture of the lateral areas (which is, as a rule, much stronger than that of the median area) will generally be found to resemble that of the anterior valve, which has no proper median area. In the posterior valve the median area is very small, while the sculpture of the rest of the valve corresponds to that of the lateral areas generally (see Fig. 261).

The articulamentum of the intermediate valves is divided into two equal parts in the middle of the anterior edge, opposite to the beak, by a _sinus_. Each of the portions thus formed is again divided by a _notch_ or suture into two unequal parts, the anterior of which is known as the _sutural lamina_, and is more or less concealed by the valve in front of it, while the lateral part, or _lamina of insertion_, is entirely concealed by the girdle. The articulamenta of the anterior and posterior valves are either simple or pierced by a series of notches (Fig. 263).

The girdle of the _Chitonidae_ varies considerably in character. Sometimes its upper surface is simply corneous or cartilaginoid, with no other sculpture than fine striae, at others it is densely beset with spines or bristles, or tufted at intervals with bunches of deciduous hairs; again it is marbled like shagreen or mossy down, or covered with serpent-like scales. The width of the girdle varies greatly, being sometimes very narrow, sometimes entirely covering all the valves (_Cryptochiton_). As a rule, its outer edge is continuous, but in _Schizochiton_ it is sharply notched over the anus.

A description has already been given of the dorsal eyes in Chiton (p. 187), the nervous system (p. 202), the branchiae (p. 154), the radula (p. 228), and the generative system (p. 126).

The recent Chitons are thus classified by Dr. W. H. Dall:--

SECTION I. CHITONES REGULARES.--_Anterior and posterior valves
of similar character._

A. _Leptoidea._--Insertion plates obsolete, or, if present,
unslit; _Leptochiton_, _Hanleyia_, _Hemiarthrum_, _Microplax_.

B. _Ischnoidea._--Insertion plates sharp, smooth, fissured; with
eaves; _Trachydermon_, _Callochiton_, _Tonicella_, _Schizoplax_,
_Leptoplax_, _Chaetopleura_, _Spongiochiton_, _Ischnochiton_,
_Callistochiton_.

C. _Lophyroidea._--Insertion plates broad, pectinated,
projecting backward; _Chiton_, _Tonicia_, _Eudoxochiton_,
_Craspedochiton_.

D. _Acanthoidea._--Insertion plates thrown forward;
_Sclerochiton_, _Acanthopleura_, _Dinoplax_, _Middendorffia_,
_Nuttallina_, _Arthuria_, _Phacellopleura_.