ALIMENTARY SYSTEM — RECTUM. 
28.3 
Fig. 564. — Portion of intestinal 
canal of T esiacella halioiitiea Drap., 
showing the plexus of blood vessels 
upon its surface, highly magnified 
after Lacaze-Duthiers). 
by special organs, as in Vertebrates, but is effected by endosmosis 
through the intestinal walls into the blood contained within a plexus 
of blood vessels distributed over the 
whole surface of the alimentary canal 
and is thence carried by the blood to 
every part of tlie body, the functions of 
digestion, absorption and circulation 
being thus closely united. 
The Rectum {rectum, straight) is the 
straight terminal section of the intestine, and often differs in colour 
and character from the preceding convolute portion, being sometimes 
more muscular and palpably thicker and sacculate, as in Plunoi’bis 
corneas, or may be more slender, as in the Limaces. In our Pelecy- 
pods, in Neritina and other of the more primitive Streptoneura, the 
rectal tube in its course towards the exterior is embraced by the 
ventricle of the heart, but in Vivipara the pericardium only is pierced 
and in other groups every gradation is found leading to the final 
freedom of the rectum from all contact with the heart. 
The rectum bears upon its outer side a band of longitudinal 
muscular fibres which retract the collar, thus shortening the rectal 
tube and expelling the contents in various forms, according to the 
species and to some extent according in colour with the nature of 
the food. Ordinarily the excrementitious matters are e.xpelled in a 
vermicular or twisted shape or they may, as in Cyclostoma, assume a 
spherical form. The anus or excretory orifice is closed by a kind of 
sphincter muscle, and being always in 
association with the respiratory cavity, 
if one be present varies its position with 
the breathing organ, but its termination 
always lies in the path of the excurrent 
stream, if a special one is present. In the Pelecypods it is placed at 
the hinder end of the body above the posterior adductor, Imt in most 
Gastropods it opens more or less anteriorl)", in dextral specimens 
upon the right side of the body and in sinistral individuals on the 
left, and is sometimes placed upon the fmcal lobe, a small and 
slightly twisted outgrowth, which in Planorbis has a rich vasculariza- 
tion. In Testacelbt, whose visceral coil has become untwisted by the 
detorsion of the body, tbe anus has reverted to its primitive 
posterior position. 
Fig. 565. — Respiratory orifice of 
Agriolimax agrestis (L.), showing 
the anterior anal cleft, x 3. 
