1921.] N. Annandale & R. B. S. SewELL ; Vivipara. 



253 



cilia. The cells are relatively deep and narrow and have large, 

 deeply-staining nuclei. Unicellular glands do not occur among 

 them. Epithelium of this type extends over both surfaces of the 

 marginal processes and over the floors of the marginal and supra- 

 marginal grooves. Above the latter it gives place, but not abruptly, 

 to non-ciliated epithelium containing unicellular glands. The 

 change is gradual, the cells becoming shorter and stouter and the 

 cilia more feeble and finally non-existent. On the surface of the 

 supraniarginal ridge, however, epithelium is usually absent after 

 birth, the underlying glands being exposed on the surface. 



FiG. 15. — Vertical section through the edge of the mantle in the adult Taia 

 ehtornlis, Annandale, in a period of arrested Growth. 

 .\. The whole structure (xSoi. 

 B. Region of the supramarginal groove more highly magnified. 



b.s., blood-sinus; e., non-ciliated epithelium; e'., ciliated epithelium; »«., 

 external retractor muscle; m"., muscular network: />., pigment granule; p-g., 

 part of periostracal gland ; s.»., degenerate remains of shell-gland ; sm.gr., supra- 

 marginal groove ; sm. "., supramarginal ridge ; y.g., yellow granules. 



Connective Tissue. — Two kinds of connective tissue can be 

 distinguished in the marginal region of the mantle of the Viviparidae. 

 The bulk of the roof of the branchial chamber consists of a pecu- 

 liar kind of cells closely resembling that of which the adipose fins 

 of fishes are mainly composed and identical with those of the foot 

 of the molluscs. These cells are of very large size and of polygo- 

 nal outline (pi. iii, fig. 3). Their walls are thick, their nuclei very 

 small and they are gorged with a gelatinous substance evidenth' 

 not protoplasmic. Immediately under the epithelium of both sur- 



