5. and 6. _The Intestine, Rectum, and Anus._--The intestine, the wider anal end of which is called the rectum, almost invariably makes a bend forward on leaving the stomach. This is the case in the Cephalopoda, Scaphopoda, and the great majority of Gasteropoda. The exceptions are the bilaterally symmetrical Amphineura, in which the anus is terminal, and many Opisthobranchiata, in which it is sometimes lateral (Fig. 68, p. 159), sometimes dorsal (Fig. 67). The intestine is usually short in carnivorous genera, but long and more or less convoluted in those which are phytophagous. In all cases where a branchial or pulmonary cavity exists, the anus is situated within it, and thus varies its position according to the position of the breathing organ. Thus in _Helix_ it is far forward on the right side, in _Testacella_, _Vaginula_, and _Onchidium_ almost terminal, in _Patella_ at the back of the neck, slightly to the right side (Fig. 64, p. 157). 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. [Illustration: 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=