198 COMPARATIVE ANATOMY CHAP. 



ampulla itself also forms longitudinal folds on its inner surface, between which 

 glandular tubes open. 



The anus, in the Cephalopoda, always carries two lateral projecting appendages, 

 which are often lancet-shaped. 



The short and narrow hind-gut of the .Wr^///(/.^/r.s opens into the 

 dorsal portion of a cavity, the cloaca, which lies at the posterior end 

 of the body ; this, again, communicates with the exterior by means of a 

 ventral and very extensible longitudinal slit. Into this cloaca the 

 ducts of the genital organs, which are morphologically to be regarded 

 as nephridia, also open. 



In the Lamellibranchiti, after the hind-gut has traversed the heart, 

 it runs straight backward over the posterior adductor, to open through 

 the anus into the posterior and upper portion of the mantle cavity 

 (anal chamber). 



On the position of the anus, cf. Section V. on the arrangement of 

 the organs in the mantle cavity. 



XVII. The Circulatory System. 

 A. General. 



All Mollusca have a circulatory system ; in some divisions, 

 especially in the ('rjihu/njioi/it and some Proadbranehic^ this may attain a 

 high level of complication by the development of a closed arterial and 

 venous vascular system. The heart, as the central organ of propulsion, 

 is never wanting. It lies enclosed in the pericardium, a division of 

 the secondary body cavity ; its primitive position is median, above 

 the hind-gut. In the Lamellibranchia and Itiotocardia, it is traversed 

 by the hind-gut, in other Gastropoda it lies near it. It is always 

 arterial, i.e. it pumps the blood flowing from the respiratory organs 

 back into the body. 



In those symmetrical Molluscs in which the dorsal portion of the 

 body rises as a high visceral dome, the intestine first ascending into 

 the dome and then descending to the anus, the heart comes to lie 

 behind the hind-gut (Dentalium, Cephalopoda). 



In asymmetrical Gastropoda, its position depends upon that of the 

 pallial complex. Where the hind-gut and anus have shifted with the 

 pallial organs to the anterior side of the visceral dome, the heart also 

 lies anteriorly (Prosobranchia, Pulmonata, a few Tedibranchia). 



The heart gives rise, as a rule, to two large arteries (aorta), one 

 of which runs to the head, the other to the visceral dome, to supply 

 blood to the viscera. Not infrequently they leave the heart as one 

 large vessel. Where the circulatory system is not closed, the arteries 

 sooner or later convey the blood to the primary body cavity or ccelom, 

 i.e. into the lacunar system. The venous blood is sometimes conveyed 

 along distinct vessels, sometimes in channels without proper walls into 



