36 Miscellaneous. 



derived occupy, therefore, originally the formative pole of the 

 embryo. It would be easy for me to found on this curious obser- 

 vation a theory of the neurcm as a sequel to the gastrcea of M. 

 Hack el. The neurula would be a gastrula which would possess, at 

 the pole opposite to that occupied by the aperture of invagination, 

 cells destined to become the central nervous system and the eyes ; it 

 might be compared to the Ctenophores in the adult state, as well as 

 to the embryos of many of the higher animals ; but I have not any 

 predilection, I must confess, for hypotheses of this kind. 



The primitive mouth soon penetrates into the interior of the 

 embryo ; and the neighbouring parts of the ectoderm afterwards 

 follow it, constituting an infundibulum which becomes the oesophagus 

 with the sac of the radula. At the bottom of this infundibulum 

 there is a fine ciliated canal, by which it communicates with the 

 cavity of the inner lamella. This canal corresponds to the primitive 

 mouth, which does not close up at any moment. This observation, 

 so easy to verify in Firoloides, sufficiently refutes the opinion of 

 certain phylogenists who believe that the primitive aperture of in- 

 vagination in the Gasteropods becomes the anus, and annihilates all 

 the conclusions that they have drawn from this supposition. It is 

 by this ciliated canal that the albumen of the egg penetrates into 

 the digestive cavity, or the primitive cavity of invagination. The 

 cells of the inner lamella absorb this albumen, and deposit it in their 

 interior under the form of strongly refracting masses, which I shall 

 name the dcutolecith. It is nevertheless only at the ventral part of 

 the ectoderm that this storing of nutritive substance takes place, the 

 rest of the lamella preserving its character of embryonic cells. At 

 its aboral part it furnishes a hollow prolongation, which unites 

 with the ectoderm below the foot to form the intestine and the 

 anus. 



The prcconchylian invagination becomes filled with a viscous 

 brownish substanco ; then it spreads out, and the viscous substance 

 extends into a thin layer, which hardens on contact with the sea- 

 water, and constitutes the apex of the shell. 



The otocysts arc formed by invagination of the ectoderm on the 

 sides of the base of the foot. The cerebroid ganglia detach them- 

 selves from the internal surface of the part of the ectoderm circum- 

 scribed by the velum, the same which afterwards gives origin to tho 

 tentacles. 



The ventral part of the entoderm forms a sac, which occupies the 

 apex of the shell ; it is the nutritive sac. The rest of the walls of 

 the embryonic digestive cavity gives origin directly to the intestine 

 and stomach, which remains in communication with the nutritive 

 sac by a large aperture. After hatching, the dcutolecith contained 

 in the w r alls of the nutritive sac becomes disaggregated, and falls 

 into the stomach to serve for the nourishment of the larva. This 

 sac afterwards acquires a lobed form, and gives origin directly to the 

 liver. 



The retractor muscle originates in a small number of cells, which 



