MOLLUSCA. 



233 



Gasteropoda. To illustrate the development of the Gasteropoda I 

 have given a detailed description of two types, viz. Nassa mutabilis and 

 Paludina vivipara. 



Nassa mutabilis. This form, the development of which has been 

 very thoroughly worked out by Bobretzky (No. 242), will serve as an 

 example of a marine Gasteropod with a large food-yolk. The segmentation 



FIG. 104. SEGMENTATION OF NASSA MUTABILIS. (From Bobretzky.) 

 A. Upper half divided into two segments. B. One of these has fused with the 

 large lower segment. C. Four small and one large segment, one of the former fusing 

 with the large segment. D. Each of the four segments has given rise to a fresh 

 small segment. E. Small segments have increased to thirty-six. 



has already been described, p. 102. It will be convenient to take up the de- 

 velopment at a late stage of the segmentation. The embryo is then formed 

 of a cap of small cells which may be spoken of as the blastoderm resting 

 upon four large yolk-cells of which one is considerably larger than the 

 others (fig, 104 A). The small and the large cells are separated by a 

 segmentation cavity. The general features at this stage are shewn in 

 fig. 105 A, representing a longitudinal section through the largest yolk- 

 cell and a smaller yolk- cell opposite to it. The blastoderm is for the most 

 part one cell thick, but it will be noticed that, at the edge of the blastoderm 

 adjoining the largest yolk-cell, there are placed two cells underneath 

 the edge of the blastoderm (me). These cells are the commencement of 

 the mesoblast. In the later stages of development the blastoderm con- 

 tinues to grow over the yolk-cells, and as it grows the three smaller yolk- 

 cells travel round the side of the largest yolk-cell with it. As they do 

 so they give rise to a layer of protoplasmic cells (fig. 105, hy) which form 

 a thickened layer at the edge of the blastoderm and therefore round the 



