SSL E. RAY LANKESTER. 



does not exhibit the bilobed character which is so early 

 assumed by the archenteron of Pisidiuni and of Limnaeus. 



Stage 7. — In figs. 13, 14, and 16 an embryo is represented 

 which has grown to double the size of that drawn in figs. 

 10, 11, 12, and has not only relatively diminished the size of 

 its velum and increased the size of its foot, but ,has also 

 developed a mouth and stomodaeum. This stage corresponds 

 to the well-known Veliger of other Gastropod ontogenies, 

 and by the development of the shell and pallial region, by 

 the further increase of the foot, reduction of the velum and 

 growth of tentacles, the embryo passes from it to the adult 

 form, as may be well seen in Leydig's excellent figures. 



The ^development of the nervous system, organs of sense, 

 and of the parts and appendages of the alimentary canal to 

 which the archenteron gives rise as well as the ovary and 

 testis, require further study than I have yet given them in 

 this Mollusc, The stage to which figures 13 and 14 bring 

 the development are sufficient for my present purpose, which 

 is to demonstrate that the blastopore is coincident with the 

 anus and that the mouth develops quite independently and 

 comparatively late. 



In the embryo drawn in fig. 14 the same parts are present 

 as in that represented in fig. 12, the larger size of the foot 

 and the smaller size of the velum being the most striking 

 changes of their common parts. But in the later embryo we 

 find in addition to this that the upper end of the archenteron 

 has become bifid, and received into its grooved extremity a 

 deep and wide invagination of the ectoderm, which is plainly 

 enough the stomodaeum, or embryonic pharynx. Of this 

 structure there is no trace in the younger phase, and there 

 can be no possible doubt of its entire independence of the 

 blastopore. Equally impossible is it to doubt that the blas- 

 topore has all along persisted as the one orifice of the arch- 

 enteron situated near the posterior pole of the embryo, alto- 

 gether remote from the site of the mouth. 



In the embryo of fig. 14 the tissue between ectoderm 

 and endoderm has increased in abundance, consisting now 

 of numerous fusiform cells, some of which have two delicate 

 branches at one pole. The coelomar surface of both the 

 ectodermal and endodermal cells is also now closely invested 

 by cells similar to the delicate up-standingfusiform corpuscles, 

 which stretch across the coelom or body cavity. These in- 

 vesting tunics of mesoderm or mesoblastic tissue (the hypo- 

 deric and hypenteric cell-layers) consist of delicate flattened 

 corpuscles which are connected by their processes with the 

 traversing fusiform corpuscles. 



