208 GASTROPODA. 



A still further specialisation of the enteron along the lines seen in 

 Fusus is found in the egg of Na**a which is still more richly supplied 

 with yolk. The formation of the germ-layers in this egg has already 

 heen described (p. 116). The entoderm is found here as a slightly 

 developed single layer of cells on the ventral side of the embryo. 

 The stomach and rudiment of the intestine appear when the massive 

 food-yolk which, at first, presses closely upon the entoderm, separates 

 from it (Fig. 95 C and D). Owing to this origin of the enteron, its 

 lumen is here also directly bounded on one side by the yolk, which, 

 even at a later time, is very extensive (Fig. 61 D and E, p. 150), 

 and fills the whole of the posterior part of the body. The intestine 

 still appears open towards the yolk-mass (Fig. 63, p. 152) and, in its 

 farther development, no doubt follows the same course as that of 

 Fusus. 



The nutritive substance is, as we have seen, stored up in various 

 parts of the entoderm, and seems frequently to influence the develop- 

 ment of the liver. It is inherently probable that the liver originates 

 from definite parts of the entoderm, always appearing in the same 

 region of the enteron, but this process may be modified through the 

 various ways in which the nutritive mass is deposited. From the 

 different conditions found, we seem to be able to conclude with some 

 certainty that the whole of the anterior part of the enteron was 

 originally specially utilised for the storing of the nutritive material. 



The anus forms in most cases through the direct fusion of the 

 entodermal intestine with the surface of the body, though some 

 authors (Wolfson, No. 131 ; P. Sarasin, No. 101 ; Jourdain, No. 

 49, etc.) speak of the development of a proctodaeum. As the latter 

 is said to occur in other Molluscs, e.g., Chiton, Teredo, Eniovalva, and 

 as it is found in the Annelid larvae, the structure of which is 

 remarkably similar to that of the forms we are now considering, its 

 presence cannot be regarded as a priori improbable. In by far the 

 greater number of Molluscan embryos, however, a proctodaeum is not 

 developed. 



F. The Gills. 



The gills have been found to develop in some Prosobranchia as 

 consecutive prominences on the ectoderm. These prominences corre- 

 spond to single branchial leaflets. Mesoderm-cells enter into them 

 and form a septum in each leaflet. The gill commonly seems to 

 appear only after the mantle-cavity has formed, arising within the 

 latter (Figs. 61, k, p. 150, and 99 and 100, p. 214), but occasionally it 



