ANATOMY AND PHYSIOLOGY OF PATELLA YULGATA. 621 



for special mention. It consists of the usual elements, viz., connective tissue 

 fibres, connective tissue cells, and elastic fibres. These last are few in number, 

 and are never so distinctly definable as in the Vertebrata. Many amoeboid cells 

 are also found among the connective tissue fibres. These are usually larger 

 than the blood corpuscles, and have long branched processes. 



The presence of connective tissue has been indicated, and its general features 

 have been pointed out under the different organs in which it occurs. 



Muscular System.- — The general muscular system, comprising the muscular 

 fibres entering into the structure of the various organs, is described under the 

 different sections where these organs are described. 



The special muscular system includes (a) the muscle of the head and neck, 

 (b) the circular muscle connecting the foot and the shell, &c, and (c) the 

 muscle of the foot proper. 



(a) The muscle of the head and neck is arranged in three layers over the 

 dorsal surface, from tentacle to tentacle, a superficial transverse, or circular, 

 layer, a middle longitudinal with numerous oblique fibres, and a deep transverse 

 layer. These layers are continuous with the muscle bands composing the foot 

 and circular muscle, and fibres from them pass up into the tentacle. On the 

 ventral aspect the middle layer is usually wanting, though a few strands of 

 oblique fibres are occasionally present. The outer and inner layers also are 

 much thicker. 



(b) The circular muscle at its origin from the shell is composed of a number 

 of plates arranged vertically, and having their long axes parallel with the 

 surface of the body. Their free ends where they spring from the shell are 

 covered with a very dense layer of epithelium, which is in direct contact with 

 the shell. The epithelium is cubical, and so closely packed that the general 

 appearance is such as to suggest that the ends of the muscle fibres are them- 

 selves in contact with the shell, and that the epithelium is really only the 

 denser terminations of the muscle fibres (PL CXLIX. fig. 6). If the epithelium 

 covering the free part of the mantle in the head region be examined, the cubical 

 nature of the epithelium is there clearly to be made out. The inner ends of 

 the cells are serrated in a manner similar to the epithelium in many other parts 

 (PI. CL. fig. 18). The free ends are covered by a thin cuticle. The cells are 

 continuous on the one hand with the epithelium covering the dorsal surface of 

 the visceral integument, and on the other with that of the dorsal surface of 

 the mantle skirt (PI. CL. fig. 20). 



The muscle plates descend vertically, branching and spreading in a fan- 

 shaped manner, so as to cause the circular muscle to be twice the breadth at 

 its union with the foot as it is at its origin from the shell. The outer lamellae 

 pass directly downwards, the inner lamellae curve round, and are continuous 

 with the muscle of the foot proper. These bands of vertical muscle are separated 



