THE INTESTINE 



241 



a. 



5. and 6. Tlte Intestine, Rectum, and Anus. — The intestine, 

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

 ul)ly makes a bend forward on leaving the stomach. This 

 is the case in the Cephalopoda, Scaphopoda, u, 



and the great majority of Gasteropoda. The /|'c\ 



exceptions are the bilaterally symmetrical 

 Amphineiira, 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 bran- 

 chial or pulmonary cavity exists, the anus is 

 situated within it, and thus varies its posi- 

 tion according to the position of the breath- 

 ins 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 Dioto- 

 cardia {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. 



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 rectimi 

 close to the anus, are present in sc^nie Proso- 

 l)ranchiata, e.g. Murex, Furpura. In the 

 Cephalopoda the anal gland becomes of considerable size and 

 importance, and is generally known as the ink-sac (Fig. 147) ; 

 VOL. Ill E 



Ink - sac of 

 Sepia, showing its re- 

 lation to the rectum : 

 a, anus ; d, duct of 

 sac ; i.g, ink - gland ; 

 z.?', portion of the sac 

 which serves as a re- 

 servoir for the ink ; o, 

 orifice of ink -gland ; r, 

 rectum ; sp, double set 

 of sphincter muscles 

 controlling upper end 

 of duct. (Modified 

 from Girod. ) 



