622 R. J. HARVEY GIBSON ON THE 



by thin plates of oblique or circularly arranged muscle, which almost entirely 

 take the place of the vertical muscle towards the exterior of the foot (PI. CL. 

 fig. 19). The fibres there are fine, and are enclosed in bundles by strands of 

 connective tissue. 



(r) In the foot proper, towards the margin there is an open connective tissue 

 network, with variously arranged branching muscle fibres scattered through it, 

 many of which are in connection with the inner ends of epithelial cells by fine 

 processes. The upper portion of the foot, which forms the floor of the visceral 

 sack, is composed of the horizontal or oblique plates of muscle continued from 

 the circular muscle. The ventral portion is composed of a network of con- 

 nective tissue fibres, amongst which are found a large number of horizontal, 

 vertical, and oblique muscle fibres. 



The connective tissue is dense on the ventral surface, especially just beneath 

 the layer of epithelium. The cells of the epithelium are continuous with those 

 of the side of the foot, but are much crushed, and often wanting in section. 

 There are no glands of any kind in the foot. 



In the head region, where there is no circular muscle, the muscle fibres 

 which sprung from the shell are continued into the free skirt of the mantle 

 (PI. CL. fig. 20). 



The individual muscular fibres of which the bands of muscle are made up, 

 are of the nonstriped variety. Each cell is a very long fibre, often as much as 

 -| of an inch in length, while the breadth is about -^y^jj of an inch. The fibres 

 do not branch, but are collected in fasciculi with a small amount of cement 

 substance between. Each fasciculus is surrounded by a small quantity of 

 connective tissue. The fibres themselves are perfectly homogeneous or faintly 

 fibrillated (PL CL. fig. 21). 



Ejndei'mal System. — The visceral dome is entirely covered by an integument 

 which is easily detachable, and which is composed of two or three layers 

 according to the position. 



Externally (PI. CXLIX. fig. 4) is a layer of dark pigment cells, then a 

 layer of connective tissue, and internally (in those parts where it covers the 

 nephridia) a layer of light pigment cells. 



The external jngmented cell layer consists of a single layer of tabular or 

 cubical cells, each containing a round or elliptical nucleus, with one or two 

 nucleoli. On vertical section, the outlines of the cells can be distinctly made out, 

 but they are not so easily seen on surface view. In such a surface view as that 

 represented in PI. CXLIX. fig. 5, the nucleus is seen to be surrounded by a 

 quantity of pigment in the form of rounded black granules. In other situations 

 the granules do not surround the clear nucleus, but lie between it and the 

 upper part of the cell, which is hyaline in appearance, but possesses no cuticle. 

 As the edge of the dome is reached, the pigment is wanting, and there appears 





