232 RENAL ORGANS. 



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

 a considerably later period thau 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 larva? 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 subse- 

 quently 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. 



The provisional organs consist of either (1) an external paired 

 mass of excretory cells or (2) an internal organ provided with a duct, 

 which is not in all cases certainly known to open externally. The 

 former structure is found especially in the marine Prosobranchiates 

 (Nassa, etc.) where it has been fully studied by Bobretzky. It consists 

 of a mass of cells on each side of the body, close to the base of the 

 foot, and not far behind the velum. This mass grows very large, 

 and below it may be seen a continuous layer of epiblast. The 

 cells forming it fuse together, their nuclei disappear, and numerous 

 vacuoles containing concretions arise in them. At a later stage 



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

 contraction there is due to the contractions of the foot. 



