PROTO-MOLLUSCS 341 



gilled is more primitive than the two-gilled condition, and there 

 are some Chitons which have a limited number of gills far back 

 on each side. As to the shell, we find eight overlapping plates 

 situated on the dorsal side. 



The remaining Proto-molluscs (fig. 200) are unfamiliar marine 

 forms inhabiting moderately or very deep water. They possess 

 no shell, but the skin is beset with calcareous spicules, and they 

 are more or less worm-like in appearance. The foot is either 

 a narrow ridge placed in a longitudinal groove (Neomenia, Pro- 

 neomenia), or may be entirely absent (Chcetoderma). There is a 

 small posterior mantle-cavity into which the intestine and paired 

 kidneys open, while it may shelter a pair of plume-like gills 

 (Chcetoderma}, or these may be represented by a tuft of filaments 

 (Neomenia) or merely by folds (Proneomenia). Though at first 

 sight the view is tempting that these genera are more primitive 

 than Chiton, and give an indication of how Molluscs might arise 

 from worm-like forms, it is more probable that they are simplified 

 animals which have lost some of the typical molluscan characters, 

 and are, so to speak, going downhill rather than uphill. 



