SHELLS OF MOLLUSCS. 247 



The mention of Nautili reminds me that these young mol- 

 luscs, which are without vestige of a shell in their mature 

 stage, are all provided with a good-sized shell in their 

 embryonic stage. According to the principles of Agassiz 

 and others, which would make embryology the principal 

 guide in zoological classification, this transitory presence of 

 the shell would imply that the naked molluscs were higher 

 in organisation than molluscs with shells. This conclusion 

 will not, I think, be accepted. But the fact that the embryo 

 has a shell, of which it is subsequently destitute, is inter- 

 esting in the speculations it suggests, and will one day, 

 doubtless, receive its due place in science. Curious it is to 

 think of the huge shell of the Whelk or Limpet fading off 

 into the small shell-plate concealed beneath the skin of the 

 Sea-hare and the Pleurohranchus, and disappearing alto- 

 gether from the Doris and Eolis. Yet perhaps not altogether 

 disappearing ; for may not those spiculae which are so abun- 

 dant in the integument of the Doris represent the shell in a 

 rudimentary condition ? I say " represent," meaning thereby 

 that the spiculse are the analogous product of secretion, not 

 the homologous " skeleton ; " for although these spiculag may 

 stiflFen the integument, and in so far fulfil a protective office, 

 I find them in other places — for example, in the mem- 

 brane which lies next the " brain." 



To discover a new animal is surely a legitimate pride. We 

 are pleased if among sand-numerous " varieties " we can 

 alight upon even a new variety, and affix our names to it ; 

 but a new animal — something no prying zoologist has ever 

 seen the like of before, something no " plodding German " 



