In the rhipidoglossate section of the Diotocardia (Trochus, Haliotis, etc.) the rectum passes through the ventricle of the heart, a fact which, taken in conjunction with others, is evidence of their relationship to the Pelecypoda.
Fig. 147.—Ink-sac of Sepia, showing its relation to the rectum: a, anus; d, duct of sac; i.g, ink-gland; i.r, portion of the sac which serves as a reservoir for the ink; o, orifice of ink-gland; r, rectum; sp, double set of sphincter muscles controlling upper end of duct. (Modified from Girod.)
In nearly all Pelecypoda the intestine is very long and convoluted, being sometimes doubled forward over the mouth. Towards its terminal part it traverses the ventricle of the heart, except in Ostrea, Anomia, Teredo, and a few more. The anus is always at the posterior end of the animal, adjacent to and slightly above the adductor muscle.
Anal glands, which open into the rectum close to the anus, are present in some Prosobranchiata, e.g. Murex, Purpura. In the Cephalopoda the anal gland becomes of considerable size and importance, and is generally known as the ink-sac (Fig. [147]); it occurs in all known living genera, except Nautilus. The ink-sac consists of a large bag generally divided into two portions, in one of which the colouring matter is secreted, while the other acts as a reservoir for its storage. A long tube connects the bag with the end of the rectum, the mouth of the tube being controlled, in Sepia, by a double set of sphincter muscles.
The Kidneys
The kidneys, nephridia,[331] renal or excretory organs, consist typically of two symmetrical glands, placed on the dorsal side of the body in close connexion with the pericardium. Each kidney opens on the one hand into the mantle cavity, close to the anus (see Fig. [64], p. 157), and on the other, into the pericardium. The venous blood returning from the body passes through the vascular walls of the kidneys, which are largely formed of cells containing uric acid. The blood thus parts with its impurities before it reaches the breathing organs.
The kidneys are paired in all cases where the branchiae are paired, and where the heart has two auricles, i.e. in the Amphineura, the Diotocardia (with the exception of the Neritidae), the Pelecypoda, and all Cephalopoda except Nautilus, which has four branchiae, four auricles, and four kidneys. In other Gasteropoda only one kidney survives, corresponding to the left kidney of Zygobranchiate Gasteropods.
Besides their use as excretory organs the kidneys, in certain groups of the Mollusca, stand in very close relation to the genital glands. In some of the Amphineura the generative products, instead of possessing a separate external orifice of their own, pass from the genital gland into the pericardium and so out through the kidneys (see Fig. [61] C, D, p. 154). In the Diotocardia it is the right kidney alone which serves, besides its excretory functions, as a duct for the emission of the generative products, the left kidney being at the same time greatly reduced in size. Thus in Patella the left nephridium is small, the right being much larger; both function as excretory organs, but the right serves as a mode of conveyance for the seminal products as well. In certain Pelecypoda (e.g. Yoldia, Avicula, Modiola, Pecten, Spondylus) the genital glands communicate directly, and with a similar object, with the renal pouch on the same side of the body, but in the majority of cases the orifices are distinct.