16 



obvious the fact that this peculiar region is composed of 

 numerous compound glands. 



The groove is, in fact, nothing but a slit-like common 

 opening of a very large number of glands. The foot in 

 the region of the groove is bounded, as elsewhere, by a 

 layer of deep epithelial cells with characteristic striated 

 cuticular margin and cilia. These ciliated cells are 

 compressed laterally and separated by gland cells. 

 Exactly the same type of cell lines the groove, and there 

 is no change even at the bottom of it, except that the 

 cilia are longer and much better developed than else- 

 where. Here the epithelial cells are much compressed 

 and the nuclei are drawn out into long spindles. 



Between the cells open the compound glands. Below 

 the epithelium is the muscular tissue, built up mainly of 

 transverse running fibres, so that their cut ends appear in 

 section (fig. 10, Fed. mus.). There are, however, scattered 

 oblique and longitudinal fibres. In the region of the 

 groove the muscular tissue remains absent, and its place 

 is taken by the glands. Thus the very distinct demarca- 

 tion of the gland is due as much to the absence of muscles 

 here as to the presence of the gland cells. The glandular 

 tissue (fig. 10, Fed. gl.) is very characteristic. There is 

 no blue staining contents characteristic of the mucous 

 gland cells or similar cells in the mantle and pedal 

 epithelium. Instead, there are packets of very thin- 

 walled cells situated some considerable distance below the 

 epithelium. 



The packets are bounded by very thin connective 

 tissue sheaths, but this is so delicate that it may seem no 

 more than the outer walls of the cells which are arranged 

 to form the packet. 



The cells are intensely vacuolated. From each 

 packet a narrow path of the same cells runs to the 



