216 GASTROPODA. 



the primary body-cavity, and are thus at first quite independent of 

 the heart. We have already repeatedly spoken of embryonic or 

 larval blood-sinuses, some of which, being capable of carrying on 

 rhythmical movement, have been assumed to be larval hearts. The 

 rudiments of the vessels first appear as such blood-sinuses of different 

 sizes ; in Pa/iuU'na, for instance, a large sinus is found beneath the 

 intestine (Fig. 88 B, ?/x, p. 194). The gradual narrowing of these 

 spaces, which are surrounded by a layer of flat cells, and their con- 

 nection with the open ends of the heart gives rise at the end of the 

 ventricle to tlue aorta and at that of the auricle to the efferent 

 branchial vein. The other vessels arise in a corresponding manner. 



The heart, in the Gastropoda, forms in a less primitive way than in the 

 Lamellibranchia (p. 76). This is not surprising, since the circulatory and 

 respiratory organs of the Gastropoda have undergone far-reaching alterations 

 in consequence of the asymmetrical shape of the body. The presence of two 

 auricles, however, and the perforation of the heart by the alimentary canal 

 in a few Prosobranchs (Diotocardia) point to conditions resembling those 

 found in the Lamellibranchs. We might even believe that the heart in both 

 divisions arose ontogenetically in a similar way, and might then consider the 

 region at which the heart formed in the pericardium as the boundary between 

 the two coelomic sacs. 



It. is an interesting fact that we have, persisting throughout life, in 

 Dentalium, a condition similar to that seen in the developing heart in the 

 Gastropoda, which, as we have seen, arises as an infolding of the pericardium. 

 According to PLATE (Solenoconch. Lit., No. 3), the heart of Dentalium 

 represents a sac-like invagination of the pericardium, and the blood-vessels 

 also are found in a condition similar to that in Gastropod embryos, being 

 mere spaces in the mesoderm between the other organs. The structures re- 

 garded by PLATE as pericardium and heart, however, are but slightly developed, 

 and the nephridia are not connected with the pericardium. It is well known 

 that Dentalium is a form already highly differentiated, but it may be possible 

 that in this respect a primitive character is retained. It appears also that, 

 among the Amphineura, the Soleuogastres show a similar primitive condition, 

 while, in the Chitonidae, the circulatory system is much more highly organised, 

 the heart being entirely surrounded by the pericardium and provided with 

 efferent and afferent vessels. 



The different positions assumed by the heart in the various 

 divisions of the Gastropoda, which are considered of great systematic 

 significance, are connected with the shifting of the different regions 

 of the body to which allusion has already repeatedly been made 

 (p. 144). One of the auricles, as was seen, is almost always lost in 

 the process. If the pallial complex is only displaced to the side, the 

 gill lies behind the heart, the auricle behind the ventricle (this is 

 notably the case in the Opisthobranchia) ; but, if the pallial com- 



