MOLLUSCA. 317 



central nervous system as a circum-oesophageal ring, of which the 

 sub-oesophageal portion may be pulled out into ventral cords; the 

 presence of a more or less developed perivisceral portion of the 

 coelom ; the dorsal position of the main vascular trunk ; an/1, finally, 

 the very general presence of a trochosphere larva, are all important 

 characters, which point to a connection more close than that which 

 ordinarily exists between groups of phyletic rank. 



On the other hand, the differences, which are also of great 

 importance, must be noted. The Mollusca are almost entirely 

 without the phenomenon of metamerism, which is so generally 

 characteristic of Annelida. It is true that signs of this phenomenon 

 are not altogether absent, for we find in Nautilus a twofold repe- 

 tition of the gills, kidneys, and auricles, and in Chiton multifold 

 repetition of the shell-plates and gills. But these are isolated 

 instances, and not generally characteristic of the group. Moreover, 

 so far as is known there is nothing in the embryo comparable to 

 the repetition of the mesoblastic somites of the Annelids the 

 phenomenon by which the adult metamerism is preceded, and on 

 which it is based. Finally, whereas in Annelids the perivisceral 

 cavity is entirely furnished by coelom, and the haemocoel is almost 

 completely canalicular; in Molluscs vascular sinuses are well developed, 

 and in some parts of the body so large that they constitute perivisceral 

 cavities to the organs in relation with them. Thus in Molluscs part 

 of the body-cavity is coelomic the portion called pericardial or viscero- 

 pericardial and part is haemocoelic ; but it must be carefully borne 

 in mind that the coelomic portion of the body-cavity is entirely 

 separate from the haemocoelic, and that, however much they may 

 appear to resemble each other in the adult, their relations and de- 

 velopment are totally different. 



The asymmetry and distortion found in the class Gastropoda is 

 a very interesting phenomenon, and in some respects comparable to 

 the still more remarkable distortion from the original symmetry 

 found in the Echinodermata. But the two phenomena differ in 

 their mode of occurrence in certain important respects, which it 

 may be desirable to point out. In Echinodermata not only is the 

 distortion completely carried out in every living member of the 

 group, but a new symmetry has in all cases been obtained, and 

 sometimes (certain Echinoidea) a second time lost. Therefore, in 

 order to arrive at an understanding of the course which the dis- 

 tortion has taken, we are entirely dependent upon embryology; 

 indeed, were it not for the knowledge gained by a study of the 



