228 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



MUSCULAR SYSTEM. 



The muscles in Aplysia are usually arranged in elongated 

 bundles. The individual fibres are tubular, and are not trans- 

 versely striated. (Fig. 21, 1, ped. m.) The chief groups are : — 



1. The muscles of the gut, extrinsic and intrinsic. 



2. The retractor muscles of the mantle, penis, etc. 



3. The muscles of the kidney, ctenidium, etc. 



4. The muscles of the foot and body wall. 



The extrinsic muscles of the gut comprise those holding 

 the buccal mass in place and the sphincter muscles at mouth 

 and anus. The first named consist of numerous straight bands 

 radiating from the buccal mass towards the foot and body 

 wall. The remainder of the gut is held in place by connective 

 tissue only. The whole alimentary canal possesses an intrinsic 

 muscular coat of fibres varying in thickness in the different 

 regions. It is thickest and most complicated in the wall of 

 the buccal mass and gizzard. (Vide Alimentary Canal.) 



Stout bands of fibres arising from the parapodia act as 

 retractors of the mantle and so of the visceral hump. There 

 are two of these bands on the right side arising from the right 

 parapodium, and one on the left side arismg from the left 

 parapodium. The right anterior muscle threads its way 

 between the vesicles of the opaline gland, passes upwards 

 between the pericardium and the left wall of the pallial cavity, 

 through the spongy mantle tissue to the surface of the mantle 

 lying beneath the shell. It arrives at the surface between the 

 kidney on the left and the free edge of the mantle containing 

 the purple glands on the right, spreading out to be inserted 

 directly on the mantle epithelium. (Fig. 15, ret. mant.) The 

 posterior right retractor muscle passes upwards from the right 

 parapodium in the region of the anus. The left muscle is 

 smaller than those on the right side, but is similar in distribu- 

 tion and origin. (Fig. 21, ret. mant.) 



