280 RENAL ORGANS. 



In Pteropods the heart is formed (Fol) close to the anus, but slightly 

 dorsal to it (fig. 108, h). The pericardium is formed from the mesoblast at 

 a considerably later period than the heart. 



A very different account of the formation of the heart is given by 

 Biitschli for Paludina. He states that there appears an immense contrac- 

 tile sack on the left side of the body. This becomes subsequently reduced 

 in size, and in the middle of it appears the heart, probably from a fold 

 of its wall. The original sack would appear to give rise to the pericardium. 



In connection with the vascular system mention may be 

 made of certain contractile sinuses frequently found in the larvae 

 of Gasteropoda and Pteropoda. One of these is placed at the 

 base of the foot, and the other on the dorsal surface within the 

 mantle cavity immediately below the velum 1 . The completeness 

 of the differentiation of these sinuses varies considerably; in 

 some forms they are true sacks with definite walls, in other cases 

 mere spaces traversed by muscular strands. They are found in 

 the majority of marine Gasteropods, Heteropods and Pteropods. 

 In Limax a large posteriorly placed pedal sinus is well developed, 

 and there is also a sinus in the visceral sack. The rhythmical 

 contraction of the yolk-sack of Cephalopods appears to be a 

 phenomenon of the same nature as the contraction of the foot 

 sinus of Limax. 



In Calyptraea (Salensky) there is an enormous provisional 

 cephalic dilatation within the velum which does not appear to be 

 contractile. Similar though less marked cephalic vesicles are 

 found in Fusus, Buccinum and most marine Gasteropods. 



In Cephalopods the vascular system is formed by a series of 

 independent (?) spaces originating in the mesoblast, the cells 

 around which give rise to the walls of the vessels. The branchial 

 hearts are formed at about the time at which the shell-gland 

 becomes closed. The aortic heart (fig. 127, c) is formed of two 

 independent halves which subsequently coalesce (Bobretzky). 



The true body cavity arises as a space in the mesoblast sub- 

 sequently to the formation of the main vascular trunks. 



Renal organs. Amongst the Gasteropods and Pteropods 

 there are present provisional renal organs, which may be of two 

 kinds, and a permanent renal organ. 



1 Rabl holds that there is no contractile dorsal sinus, but that the appearance of 

 contraction there is due to the contractions of the foot. 



