APLYSIA. 203 



A portion of the mantle is marked off by its opacity 

 from the remainder. This portion is in the form of a band 

 about an eighth of an inch wide which closely follows the 

 contour of the shell. It is the shell-forming region of the 

 mantle, and owes its opacity to the greater height of its 

 unciliated columnar cells and to their abundant contents. 

 (See Fig. 2l,sh.f.) 



Through the transparent skin can be seen the kidney, the 

 liver, and the spongy tissue overlying the purple gland. 

 (Cf. Figs. 15, 21.) The kidney is a triangular gland which is 

 asymmetrically placed on the left side of the visceral mass. 

 Its surface is channelled by the blood-vessels belonging to the 

 afferent renal system. On the right of the kidney lies spongy 

 tissue, forming the thickness of the mantle shelf at this point. 

 Between the spongy tissue and the kidney, in the acute angle 

 between the two, a stout band of muscle (Fig. 15, ret. mani.) 

 makes its appearance. This band, when traced in sections, 

 is found to arise from the base of the right parapodium. It 

 threads its way between the pericardial cavity and the anterior 

 aorta, and is inserted on the under-side of the mantle beneath 

 the right side of the shell. This muscle probably functions 

 as a retractor of the visceral hump. A similar but much 

 smaller muscle occurs on the left side, arising from the left 

 parapodium. (Vide Muscular System.) 



Posteriorly the space beneath the mantle is occupied by 

 the liver. 



Histology of the Foot and its Glands. 



The pedal epithelium consists of slightly pigmented cells 

 interspersed between which are goblet-shaped mucous gland 

 cells. The pedal sole is ciliated throughout, and there is an 

 abrupt line of division between these cells and the smaller and 

 less compact, strongly pigmented, unciliated cells of the lateral 

 body wall. Beneath the epithelium the cells are very irregular, 



