528 A. -M. (;AKI{ .SAUNDKI^S and MAUGARET POOLE. 



Molluscs. For in the latter phylum there is never any trace 

 of seg'mentation, whereas in the former, as is well known, the 

 bands become split up into blocks, in each of which a 

 coelomic cavity is formed. A further difference is the reduc- 

 tion of the coelom in Molluscs, but this is by no means so 

 great as is usually supposed. Though the evidence is as yet 

 scanty owing to the small amount of work that has been done 

 on the later stages on development, nevertheless there is 

 reason to believe that the coelom is, at a late period in the 

 metamorphosis, of considerable size, and tliat even in the 

 adults of some of the more primitive groups it remains large. 

 Kowalevsky long ago described the development of the cceloin 

 iji Chiton polii, and some of his figures, wiiich show the 

 ccfilom surrounding the gut, would pass well for a transverse 

 section of an Annelid larva at a late stage. It is very probable 

 that a similarly extensive development of the ccelom will be 

 found in the Solenogastres, where, as we have seen, the 

 mesoderm bands are of considerable size. Among the 

 Aspidobranchia, as the most primitive of the Gastropoda, we 

 might expect a larger body cavity than in the more special- 

 ised forms, if one regards the extensive coelom as a primitive 

 factor preserved from an Annelid ancestor ; but unfortunately 

 nothing is known about the later larval stages. In a recent 

 account of the structure of the Neritidte, Bourne, hovrever, 

 has lately described a very lai-ge coelom, more extensive than 

 has been described in any other gastropod. To find a parallel 

 to it we must refer, he says, to the Cephalopoda. There are 

 a number of descriptions of the development of the Pectini- 

 branchia, in which the ctelom is extensive ; all the authoi's 

 who have worked on Paludina agree upon this. Other cases 

 are Vermetus, where Salensky describes a somatopleur and a 

 splanchnopleur, though unfortunately he gives no figures; and 

 Bithynia, described and figure by Erlanger. Among Pulmo- 

 nates andLamellibranchs, asone might expect in such special- 

 ised forms, we find no evidence of the existence of a large 

 coelom, this structure being reduced in every case to a small 

 sac-like pericardium and a reno-pericardial duct. These 



