MORPHOLOGY OF MBLIBE 543 



the tract. Only the ordinary corrugated part of the intestinal 

 lining remains. The extent of the typhlosole, however, varies, 

 because one specimen, whose intestine was sectioned, had no 

 typhlosole in the smaller part of the canal. 



The structure of the intestine is similar throughout. There 

 is an outer fibrous layer and an inner glandular one. Between 

 these are fine connective-tissue fibres and small cells and 

 colourless lymph. The glandular layer consists of tall ciliated 

 columnar epithelium (PI. 33, figs. 45-7). There is no difference 

 in the glandular layer of the typhlosole and of the remainder 

 of the intestine, or in these parts of the pyloric diverticulum. 

 A close study of the internal layer of the intestine reveals some 

 interesting morphological facts, viz. this layer consists of very 

 tall columnar cells with the nucleus located, in most cases, 

 in the middle (PI. 33, figs. 45, 47) with vacuoles either around 

 the nucleus, on either side, or on the distal side only. The 

 vacuoles arise as a confluence of smaller vacuoles which arise 

 in the neighbourhood of the nucleus, and again break up into 

 smaller ones and then pass into the lumen of the intestine. 

 The epithelial secretion does not keratinize here as it does in 

 the gizzard. Eliot (1910) suggests that the hepatic secretion 

 hardens in the intestine and the stomach of Dorids. The 

 hard substance, however, which covers the endoderm of the 

 alimentary tracts of Eliot's Dorids and of M. 1 eon in a, is 

 at least in the case of the latter of an entirely different origin. 

 Sometimes more than one nucleus may be present in the same 

 cell (PI. 33, fig. 46). The most striking feature of this epithelium 

 is the regular fibrillar structure, and the linear arrangement 

 of the cytoplasmic granules from border to base of the cell 

 running parallel with the cilia ; these do not converge on the 

 nucleus but pass to the base of the cell. The cell rests on 

 a distinctly granular basement membrane (PI. 33, fig. 45, Bm). 

 There are two distinct rows of basal granules, or terminal bar.-, 

 (desmochondria), the proximal being the larger of the two (Bg). 

 The cilia (Cil) may be seen readily between the two terminal 

 bars. Beneath the basement membrane (Bm) is a loose 

 connective-tissue layer {Lcc) which is covered by a denser 



