OR OSSEOUS SYSTEM. 



is almost always substituted for that earth, as we see also in 

 the vertebrated classes. In some of the more solid and massive 

 coverings, however, of these animals we meet with the 

 carbonate of lime, as in the serpula, the cirrhopods, and the 

 Crustacea. 



VI. Entozoa. The intestinal worms have for the most 

 part a tough exterior, transparent and almost hermogenous 

 covering spread over their whole body, to which they owe 

 their peculiar stiffness and elasticity, and to the inner surface 

 of which their cutaneous muscles are attached. This part 

 is composed of the true skin, and that epidermic covering 

 which becomes consolidated into a dense exterior skeleton 

 in higher classes. It is here soft and elastic, to allow them 

 with more ease and safety to move through the tough and 



constantly moving parts of the 

 living animals in which they 

 reside. The long cylindrical bodies 

 of these parasitic worms would be 

 impeded in their motions by any 

 ,hard, inflexible shelly covering ? 

 which likewise could not be cast 

 off and renewed in such a medium; 

 hence their smooth, glistening,and 

 unctuous covering has only that 

 degree of density and toughness, 

 which is adapted to protect them 

 from tearing and compression 

 during the movements of the living 

 parts around them. This trans- 

 parent elastic tunic is especially 

 thick and firm in the long cylin- 

 drical, filiform, or trematoid en- 

 tozoa,as thejilaria,ihe strongylus, 

 the echinorhynchus, and the as- 

 caris. Intheascaris lumbricoides, 

 which is represented in Fig. 10, A, 

 this covering is thick and tough ; 

 but so transparent as to allow the 

 white layers of muscular fibres to 

 be perceived through it. The 

 three moveable oral lobes are 



FIG. 10. 



