226 NUD1BRANCHIATE GASTEROPODA. 



trace the gradual evolution of an animal from a mass 

 of cells, each stage in the evolution presenting not 

 only its own characteristics, but those marks of 

 affinity with other animals which make the whole 

 world kin ? To watch the formation of the blood- 

 vessels, to see the heart first begin its tremulous 

 pulsations, to note how life is from the first one 

 incessant struggle and progress these keep us with 

 fascinated pertinacity at our studies/ 



The remarkable fact above mentioned, of the 

 young of the Nudibranchiate Gasteropoda being 

 furnished with a shell is exhibited not only in 

 Doris, but in Eolis, Tritonia, Aplysia, &c., while the 

 embryos of the Purpura, Nerita, Trochus, &c., are 

 likewise in their youthful state furnished with cilia, 

 by the agency of which these animals swim freely 

 about in their native element. 



There is one exception to this, which occurs in 

 Chiton, the early stage of which has recently been 

 shown by the observations of Mr. Clarke and Pro- 

 fessor Loven to be peculiar, and more resembling 

 that of an annelid than of a mollusc. In this case 

 the animal can scarcely be said to undergo a meta- 

 morphosis ; for the embryo, even within the egg, 

 has nearly the form of the parent, and the appear- 

 ance of the shell-plates is a mere matter of develop- 

 ment. 



I have never attempted to count the ova that 

 were contained in any single riband of Doris spawn, 



