VIII 



THE INTESTINE 



241 



O/ 



a. 



5. and 6. The Intestine^ Rectum^ and A71US. — The intestine, 

 the wider anal end of which is called the rectum, almost invari- 

 ably 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), some- 

 times dorsal (Fig. 67). The intestine is 

 usually short in carnivorous genera, but 

 long and more or less convoluted in those 

 wliich 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 TestaceUa^ Vagi- 

 nula, and Onchidiiim 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 Dio- 

 tocardia (^Trochus, Haliotis^ etc.) the rectum 

 passes through the ventricle of the heart, a 

 fact which, taken in conjunction with others, 

 is evidence of their relationshijo to the 

 Pelecypoda. 



In nearly all Pelecypoda the intestine is 

 very long and convoluted, being sometimes Fig. 147 

 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 Proso- 

 branchiata, 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) ; 



i.r 



Ink - sac of 

 Sopia, showing its re- 

 lation to the rectum : 

 a, anus; d, duct of 

 sac ; i.g, ink-gland ; 

 i.r, portion of the sac 

 which serves as a re- 

 servoir for the ink ; o, 

 orifice of ink-gland ; ?•, 

 rectum ; sp, double set 

 of sphincter muscles 

 controlling upper end 

 of duct. (Modified 

 from Girod.) 



VOL. ni 



