THE BODY-CAVITY, ETC. 



79 



rudiments is suggested here also, and the double character is still 

 more recognisable in the rise of the auricles, which originate as 

 invaginations of the outer walls of the pericardial vesicles (Fig. 32). 

 But this double character may, as already mentioned, be derived 

 from the connection of the formation of the heart with the paired 

 eoelomic sacs. Further, the paired character of the heart, as repre 

 sented in the adult condition, seems to us easily explained by these 

 ontogenetic processes.* The fact that, in the paired heart of Area, 



Fig. 33. — A-ll, diagrams illustrating the formation of the heart. A, in the Annelida, 

 B, in Area, C, in other Lamellibranchs. (The auricles are omitted for the sake ot 

 clearness), d, intestine ; //. paired rudiment of the heart (united, in .1 IV. and C 

 IN", to form an unpaired heart) ; //. the two pericardial vesicles (united in C IV. to 

 form the pericardium) ; so, somatic, sp. splanchnic layer of the eoelomic sac (primi- 

 tive segments). 



•there is a common anterior and posterior aorta, seems to point rather 

 to the breaking up of an originally single heart than to the union of 

 two distinct hearts. The paired dorsal vessel of the Annelida often 

 -shows connection between the two parts, j- and this also might be a 



* Grobben, who advocates such a view of the Lamellibranch heart, speaks 

 of the " retention of au ontogenetic stage through an arrest of develop- 

 ment." It appears to us also that the method of formation described would 

 facilitate the development of a double heart in cases in which such a heart 

 would be of advantage to the animal. 



t Megascolex, Microchaeta and Acanthodrilus show the recurrence of connec- 

 tions between the two hearts. In another Acanthodrilus almost the whole 

 •of the dorsal vessel is paired and is without transverse connections, but in its 

 anterior part there is still a connection. Beddard, Note on the Paired Dorsal 

 Vessel of Certain Earthworms. Proc. Roy. PJu/s. Soc. Edinburgh. Vol. viii., 

 1885. 



