Generally the hinge-line is curved, but it is sometimes straight. The beaks are mostly more or less contiguous, but they may be removed from one another .to a greater or less distance, and in some anomalous forms they are not near one another at all. In the Arcadae the two beaks are separated from one another by an oval or lozenge-shaped flat space or area. When teeth are present, they differ much in their form and arrangement. In some forms (fig. 207) the teeth are divisible into three sets - one group of one or more teeth, placed immediately beneath the umbo, and known as the "cardinal teeth;" and two groups on either side of the preceding, termed the "lateral teeth." Sometimes there may be lateral teeth only; sometimes the cardinal teeth alone are present; and in some cases (Arcadae) there is a row of similar and equal teeth.

The body in the Lamelli-branchiata is always enclosed in an expansion of the dorsal integument, which constitutes the "mantle," or "pallium," whereby the shell is secreted. The lobes of the mantle are right and left, and not anterior and posterior as are the mantle-lobes of the Brachio-poda. Towards its circumference the mantle is more or less completely united to the shell, leaving in its interior, when the soft parts are removed, a more or less distinctly impressed line, which is called the " pallial line," or "impression" (fig. 210).

There is no distinctly differentiated head in any of theLa-mellibranchiata, and the mouth is simply placed at the anterior extremity of the body. It is furnished with ciliated and tactile membranous processes or "palpi" (usually four in number), but there is no dental apparatus. The mouth opens into a gullet, which conducts to a distinct stomach. On the right side of the stomach, and opening into it, is, in many cases, a blind sac containing a peculiar transparent glassy body, which is known as the "crystalline stylet," but the functions of which are absolutely unknown. The intestine has its first flexure neural, generally perforates the wall of the heart, and terminates posteriorly in a distinct anus, which is always placed near the respiratory aperture. The liver is large and well developed, but there are no salivary glands.

Fig. 208.   Anatomy of a bivalve Mollusc (Mya arenaria). The left valve and mantle lobe and half the siphons are removed.s s Respiratory siphons, the arrows indicating the direction of the currents ; a a' Adductor muscles ; b Gills ; h Heart; o Mouth, surrounded by (p) labial palpi; f Foot; v Anus ; m Cut edge of the mantle.

Fig. 208. - Anatomy of a bivalve Mollusc (Mya arenaria). The left valve and mantle-lobe and half the siphons are removed.s s Respiratory siphons, the arrows indicating the direction of the currents ; a a' Adductor muscles ; b Gills ; h Heart; o Mouth, surrounded by (p) labial palpi; f Foot; v Anus ; m Cut edge of the mantle. "After Woodward.)

There is always a distinct heart, composed either of an auricle and ventricle, or of two auricles and a ventricle. The ventricle propels the blood into the arteries, by which it is distributed through the body. From the arteries it passes into the veins, and is conducted to the gills, where it is aerated, and is finally returned to the auricles.

The respiratory organs in all the Lamellibranchiata consist typically of two lamelliform gills, placed on each side of the body (fig. 208, b and fig. 209,g). In some cases there is only one gill on each side of the body, the external pair of branchiae being absent. The gills are in the form of membranous plates, composed usually of tubular rods, which support a network of capillary vessels, and are covered with vibrating cilia, whereby a circulation of the water is maintained over their surfaces. In some bivalves the margins of the mantle are united to one another, so that a closed branchial chamber is produced; and in the others the arrangements for the admission of fresh and the expulsion of effete water are equally perfect, though there is no such chamber. In those in which the mantle-lobes are united at their margins, there are two orifices, one of which serves to admit fresh water, whilst the effete water is expelled by the other. The margins of these "inhalant" and "exhalant" apertures are often drawn out and extended into long muscular tubes or "siphons," which may be either free, or may be united to one another along one side (fig. 208, s s), and which can usually be partially or entirely retracted within the shell by means of special muscles, called the "retractor muscles of the siphons." These siphons are more especially characteristic of those La-mellibranchs which spend their existence buried in the sand, protruding their respiratory tubes in order to obtain water, and with it such nutrient particles as the water may contain. The presence or absence of retractile siphons can be readily determined merely by inspection of the dead shell. In those bivalves in which siphons are not present, or if present are not retractile, the " pallial line " in the interior of the shell is unbroken in its curvature, and presents no indentation (Integro-pallialia). In those, on the other hand, in which retractile siphons exist, the pallial line does not run in an unbroken curve, but is deflected inwards posteriorly, so as to form an indentation or bay, which is termed the "pallial sinus," or "siphonal impression," and is caused by the insertion of the retractor muscle of the siphon. Those bivalves in which this sinus exists form the section Sinupallialia (fig. 210, 2).

Fig. 209.   Diagrammatic vertical and transverse section of Mya arenaria. b Back, or

Fig. 209. - Diagrammatic vertical and transverse section of Mya arenaria. b Back, or "dorsal margin" of the shell; .s s The two valves of the shell, right and left; m m The two halves, or " lobes," of the mantle, producing the shell; g g The gills, two pairs on each side; h The heart; i Intestine; f The foot.