SHELLS OF MOLLUSCS. 261 



the shell are seen to wave with more vigour, and the 

 animal moves quickly. Just before emerging from its 

 crystal envelojDe, the rapidity of its motion is very great; 

 and a wondrous spectacle it is to behold many hundreds 

 of them whirling and whirling about till they escape into 

 the water, where they swim to and fro like crowds of 

 tiny Nautili disporting themselves on the ocean. 



The mention of Nautili reminds me that these young 

 molluscs, 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 interesting in 

 the speculations it suggests, and will one day, doubt- 

 less, 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 disap- 

 pearing altogether from the Doris and Eolis. Yet 

 perhaps not altogether disappearing ; for may not those 

 spiculse which are so abundant in the integument of 

 the Doris represent the shell in a rudimentary condi- 

 tion ? I say " represent," meaning thereby that the 

 spicul?e are the analogous product of secretion, not the 

 homologous " skeleton ; " for although these spicula? 



