898 ANATOMY AND DEVELOPMENT 



THE MUSCULAR SYSTEM. 



The general muscular system consists of (i) the general 

 wall of the body; (2) the muscles connected with the mouth, 

 pharynx, and jaws; (3) the muscles of the feet; (4) the muscles 

 of the alimentary tract. 



The muscular wall of the body is formed of (i) an external 

 layer of circular fibres; (2) an internal layer of longitudinal 

 muscles; (3) a layer of transverse fibres. 



The layer which I have spoken of as formed of circular fibres 

 is formed of two strata of fibres which girth the body somewhat 

 obliquely (PI. 51, fig. 25). In the outer stratum the rings are 

 arranged so that their ventral parts are behind, while the ventral 

 parts of the rings of the inner stratum are most forward. Both 

 in the median dorsal and ventral lines the layer of circular fibres 

 become somewhat thinner, and where the legs are attached the 

 regularity of both strata is somewhat interfered with, and they 

 become continuous with a set of fibres inserted in the wall of the 

 foot. 



The longitudinal muscles are arranged as five bands (vide 

 fig. 1 6), viz. two dorsal, two lateral, and three ventral. The 

 three ventral may be spoken of as the latero-ventral and medio- 

 ventral bands. 



The transverse fibres consist of (i) a continuous sheet on 

 each side inserted dorsally in the cutis, along a line opposite 

 the space between the dorsal bands of longitudinal fibres, and 

 ventrally between the ventro-median and ventro-lateral bands. 

 Each sheet at its insertion slightly breaks up into separate 

 bands. They divide the body-cavity into three regions a 

 median, containing the alimentary tract, slime glands, &c., and 

 two lateral, which are less well developed, and contain the nerv- 

 ous system, salivary glands, segmental organs, &c. 



(2) Inserted a little dorsal to the transverse band just de- 

 scribed is a second band which immediately crosses the first, 

 and then passes on the outer side of the nervous cord and 

 salivary gland, where such is present, and is inserted ventrally 

 in the space between the ventro-lateral and lateral longitudinal 

 band. 



