The tentacles or papillae developed on a greater or less extent of the margin of the mantle, and at the siphonal apertures in many Lamelli-branchiata, are probably tactile. Cells ending in sensory hairs are found upon them in Pecten, Anomia, etc, as well as on the margins of the mantle1 (p. 137). Eyes provided with a lens are often found in the larva. They are two in number and lie at the base of the velum on each side of the oesophagus near the auditory sacs. Teredo has a row of such organs in front of the foot. But in the adult eyes are confined to the edges of the mantle and the siphons. They have the form of diffuse ommatidia, which are generally present and are sometimes the only kind to be found, e.g. Ostrea, Avicula, Cardita sulcata; or else the ommatidia are aggregate, and then the eye is either pseudo-lenticulate, evaginate, or invaginate. The first-named is found only in Arca; the retineum is slightly invaginate but the retinidial elements form a prominent convex lens. The evaginate type occurs in Arca and is the only kind in Pectuncidus. There is an ommateum; the retinophorae are surrounded each by eight (retinulae, four forming an outer, four an inner row, and the retinophoral rods are prominent and convex at their outer extremities.
The invaginate eye is met with in Area as a simple pit; in Pecten, Spondylus, and Cardium the pit becomes a vesicle and closely resembles the Vertebrate eye. In Arca the pit is lined by a retineum, and the retinidial cuticula is thickened.
1 For the structure of the hypodermis, the intercellular nerves, the plexus of nerve-fibrils in the inner layer of the cuticle, and the sense-organs of the tentacles in Pecten, see Patten, Mitth. Zool. Stat. Naples, vi. pp. 604-5, 662-5, with the figures given.
In Pecten the hypodermis and a thin layer of connective tissue constitute a transparent cornea over the eye, which is situate at the apex of a tentacle. The corneal hypodermic cells form a lens-like body in P. pusio. The hypodermis cells of the surface surrounding the cornea are deeply pigmented and are termed iris. Beneath the cornea is a mesoblastic cellular lens, convex internally and projecting into a blood-sinus. The optic vesicle, which is derived from an ingrowth of the hypodermis, has its anterior wall converted into a retina, in which the retinophoral rods are turned away from the light as in Vertebrata. Its posterior wall is transformed into a double-layered argentea, in which the cells are obliterated and a red coloured cellular tapetum. Between the argentea and the retinophoral rods is intercalated a thin hyaline vitreous layer secreted by the cells of the argentea (?). The vesicle is invested by a thin mesoblastic sac, which forms a septum between the lens and retina, a sclerotic external to the tapetum. The optic nerve is derived as in all Lamellibranchiata from the circumpallial nerve. It divides into two branches. One which spreads over the sclerotic is central; its fibres enter the optic vesicle near its rim and become the axial nerves of the retinophorae.
The other is lateral; its fibres pass to the anterior surface of the retina, pierce the septum, and are distributed to the ganglion cells and between the retinophorae. Spondylns appears to agree closely with Pecten; but the structure is very much simplified in Cardium 1. A pair of auditory sacs are usually present when the foot is well developed. They lie close to the pedal ganglion in connection with a nerve from it2. The central cavity contains a single calcareous otolith.
The mouth is anterior and ventral to the anterior adductor when present. It is elongated transversely, and both its margins, upper and lower, are formed by two prominent ridges which are prolonged laterally into the labial tentacles. These structures are ciliated, and often of great extent, thrown into folds or fringed. The anus is a projecting papilla placed dorsally to the posterior adductor muscle. The alimentary canal is ciliated throughout, and consists of a short oesophagus, a dilated stomach to which is often attached a caecum, and an intestine which is disposed in folds. The terminal section is straight, median, and dorsal, and usually passes through the ventricle, except in Ostrea, Anomia, and Teredo. On the crystalline style, see p. 133, ante. Two ducts open into the stomach, one on either side. They lead to a large branched gland, the liver, which with the genital glands surrounds the coils of the intestine. Calcareous cells are not present in the liver, only granular and ferment cells. The heart lies in a pericardium in front of the posterior adductor muscle, and very generally beneath the shell-ligament. The ventricle is single, except in Area, cf. Plate vii.
Fig. 3, p. 291. There are two auricles, usually separate but united medianly in Ostreidae. The ventricle gives origin to an anterior and posterior aorta. Fine vessels with an endothelial lining are found in the labial tentacles and the intestine. Otherwise the blood spaces are irregular spaces between the viscera and in the mantle. A muscular valve surrounds the large venous channel which brings the blood from the foot to the median infra-cardiac sinus, and by its contraction causes the foot to swell as it does in locomotion. A portion of the mantle appears to form a blood reservoir. The blood is colourless, and contains amoeboid corpuscles. Haemoglobin coloured corpuscles occur in Solen (Ceratisolen) legumen and Area Noe. The admixture of water with the blood and the presence of pori aquiferi on the margins of the foot appear to be very doubtful. The coloured corpuscles of Solen do not escape even when the animal is strongly irritated and consequently contracted. 'The pericardial gland' (see p. 129) 'appears in two forms, (1) as lobes upon the auricles, (2) as caecal outgrowths of pericardial epithelium into the mantle at the anterior angles of the pericardium.