rjQ LAMELLIBRANCHIA. 



the mesodermal tissues of the latter. The passage of these out of 

 the pericardium that surrounds them is, owing to the nature of their 

 origin, easily understood (Fig. 32 C and D). 



This method of formation of the heart from the mesial walls of the pericardial 

 vesicles explains how, in the adult, the intestine traverses the heart. Phylo- 

 genetically, this condition is supposed to have arisen through a blood-sinus 

 surrounding the intestine developing thicker walls and thus becoming the 

 heart (Gbobben). Since the vessels arise distinct from the heart, such an 

 origin of the latter is not in any way improbable. On the other hand the 

 fact that in the Lamellibranchia, a paired heart lying dorsally to the 

 intestine and with each half enclosed in a separate pericardium may occur 

 , irca) has led to the conclusion that the unpaired heart which, m the higher 

 forms surrounds the intestine, might have arisen by the fusion of these two 

 hearts (Thiele, Chap. xxx.). This view seemed to be supported by the fact 

 that the double heart is found in just those forms that are very primitive, and, 

 further that a double heart is also present in various Annelids. 



The paired origin of the heart (Figs. 32 and 33 C), might perhaps be regarded 

 as a primitive feature and as indicating that the heart was originally a paired 

 vessel but this view is not justified, since it is supported merely by the paired 

 development of the coelom and the part taken by the latter in the formation 

 of the heart A comparison with the manner in which the heart arises in the 

 Annelida and its formation in the Lamellibranchia should help to elucidate 

 these points (cf. Vol. i., p. 291). 



In the Annelida, the paired origin of the heart is still more 

 marked than in the Lamellibranchia. Even during the growth of the 

 primitive segments towards the dorsal middle line the rudiment of 

 the dorsal vessel appears on that side of the splanchnic layer which 

 is turned towards the entoderm (Fig. 33 A, I. and II.). The dorsal 

 vessel is therefore paired and, as the primitive segments grow further, 

 shifts towards the dorsal line (.4 II. and III.) On this line, the two 

 rudiments of the heart finally meet (A IV.) and fuse to form the 

 unpaired dorsal vessel, except in those forms in which they remain 

 distinct in the adult. With this latter condition in which the dorsal 

 vessels remain distinct, the heart of Am, shows the greatest agree- 

 ment We must suppose that, in Area, each of the two pericardial 

 sacs by the invagination of its inner wall, developed a ventricle (Fig. 

 33 B I -IV h). The fusion of the pericardial sacs above and below 

 the intestine did not take place, and in this way the union of the 

 two rudiments of the heart was prevented. In most Lamelhbranchs, 

 on the contrary, the circumcrescence of the intestine takes place: 

 the whole median wall of the pericardial vesicles takes part in the 

 formation of the ventricle, and the latter thus surrounds the intestine. 

 (Fig 33 G, I. -IV.). The rise of this single ventricle from distinct 



