This division includes those members of the sub-kingdom Mollusca in which the nervous system consists of three principal pairs of ganglia ; and there is always a well-developed heart, which is never composed of fewer than two chambers.
The Mollusca proper may be roughly divided into two great sections, respectively termed the Acephala and the Encephala (or Cephalophora), characterised by the absence or presence of a distinctly differentiated head. The headless, or Acephalous, Molluscs correspond to the class Lamellibranchiata ; also distinguished, at first sight, by the possession of a bivalve shell. The Encephalous Molluscs are more highly organised, and are divided into three classes - viz., the Gasteropoda, the Pteropoda, and the Cephalopoda. The shell in these three classes is of very various nature, but they all possess a singular and complicated series of lingual teeth; hence they are grouped together by Professor Huxley under the name of Odontophora.
The members of this class are characterised by the absence of a distinctly differentiated head, and by having the body more or less completely protected in a bivalve shell. There are one or two lamellar gills on each side of the body, the intestine has a neural flexure, and there is no odontophore.
The Lamellibranchiata are commonly known as the "bivalve" shell-fish, such as Mussels, Cockles, Oysters, Scallops, etc, and they are all either marine or inhabitants of fresh water.
Though they agree with the Brachiopoda in possessing a shell which is composed of two pieces or valves (small accessory plates are present in Pholas, &c), there are, nevertheless, many points in which the shell of a Lamellibranch is distinguished from that of a Brachiopod, irrespective of the great difference in the structure of the animal in each. The shell in the Brachiopoda, as we have seen, is rarely or never quite equivalve, and always has its two sides equally developed (equilateral); whilst the valves are placed antero-posteriorly as regards the animal, one in front and one behind, so that they are "dorsal" and "ventral." In the Lamellibranchiata, on the other hand, the two valves are usually of nearly equal size (equivalve), and are more developed on one side than on the other (inequilateral); whilst their position as regards the animal is always lateral, so that they are properly termed "right" and "left" valves, instead of "ventral" and "dorsal." It is to be remembered, however, that many of the Bivalves, such as the Oysters, habitually lie on one side, in which case the valves, though really right and left, are called "upper" and "lower." It is to be borne in mind also, that the two valves, especially in the attached Bivalves, may be very unsymmetrical, one valve being much larger or deeper than the other. Lastly, there are some cases (e.g., Pectunculus) in which the shell becomes very nearly equilateral, the line drawn from the beaks to the base dividing the shell into two almost equal halves.
Fig. 207. - Left valve of Cytherea chione. (After Woodward.) A, Anterior margin. B, Posterior margin. C, Ventral margin or base ; u Umbo ; h Ligament; l Lunule; c Cardinal tooth; t t Lateral teeth; a Anterior adductor; a' Posterior adductor; p Pallial line; s Pallial sinus, caused by the retractor muscles of the siphons.
The following are the chief points to be noticed in connection with the shell of any Lamellibranch (fig. 207): Each valve of the shell may be regarded as essentially a hollow cone, the apex of which is turned more or less to one side; so that more of the shell is situated on one side of the apex than on the other. The apex of the valve is called the "umbo," or "beak," and is almost always turned towards the mouth of the animal. Consequently, the side of the shell towards which the umbones are turned is the "anterior" side, and it is usually the shortest half of the shell. In some Bivalves, however, the beaks are "reversed," and are turned towards the posterior side of the shell. The longer half of the shell, from which the umbones turn away, is called the "posterior" side, but in some cases this is equal to, or even shorter than, the anterior side. The side of the shell where the beaks are situated, and where the valves are united to one another, is called the "dorsal" side; and the opposite margin, along which the shell opens, is called the "ventral" side, or "base." The length of the shell is measured from its anterior to its posterior margin, and its breadth from the dorsal margin to the base.
At the dorsal margin the valves are united to one another, for a shorter or longer distance, along a line which is called the "hinge-line." The union is effected in most shells by means of a series of parts which interlock with one another (the " teeth "), but these are sometimes absent, when the shell is said to be "edentulous." Posterior to the umbones, in most Bivalves, is another structure passing between the valves, which is called the " ligament," and which is usually composed of two parts, either distinct or combined with one another. These two parts are known as the " external ligament" (or the ligament proper) and the " cartilage," and they constitute the agency whereby the shell is opened, but one or other of them may be absent. The ligament proper is outside the shell, and consists of a band of horny fibres, passing from one valve to the other just behind the beaks, in such a manner that it is put upon the stretch when the shell is closed. The cartilage, or internal ligament, is lodged between the hinge-lines of the two valves, generally in one or more " pits," or in special processes of the shell. It consists of elastic fibres placed perpendicularly between the surfaces by which it is contained, so that they are necessarily shortened and compressed when the valves are shut. To open the shell, therefore, it is simply necessary for the animal to relax the muscles which are provided for the closure of the valves, whereupon the elastic force of the ligament and cartilage is sufficient of itself to open the shell.