MUSCULAR SYSTEM. Ml 



FOURTH SECTION. 



Muscular System of the Cyclo-Ganliated, or Molluscous 

 Classes. 



The limited extent and the aquatic nature of the respira- 

 tion in most of the molluscous classes, and their fixed con- 

 dition, or imperfect means of locomotion, are accompanied 

 with a corresponding low degree of vitality and imperfect 

 development of their muscular system. The forms also of 

 this system, as of the whole body, vary much more in this 

 than in the articulated or vertebrated divisions of the 

 animal kingdom. Within the exterior cartilaginous covering 

 of the tunicated animals is placed their muscular coat by which 

 they are able to contract their whole body, and forcibly 

 to expel the contents of their respiratory or thoracic cavity. 

 This muscular enveloping tunic, as seen in that of the cyn- 

 thia pupa (Fig. 65,) consists chiefly of long diverging fasciculi 

 which originate from around the two orifices of the sac (a. b,) 

 and extend round the whole body of the animal. These 

 muscular fasciculi are attached to various points of the 

 exterior tunic, but most intimately around the respiratory 

 (#,) and the anal ( b, ) 



orifices, which we ob- FIG - 65> 



serve also to be provided 

 with distinct and strong 

 sphincter muscles (d, e,) 

 passing in a circular man- 

 ner around them. By 

 the contraction of the 

 whole of this muscular 

 tunic, the orifices are 



