TISSUE-DIFFERENTIATION 



113 



an extent that the whole body is inclosed in a heavy 

 bony box. In the mollusks (shellfish, snails, etc.) 

 this sort of protective structure has taken a different 

 line of development through the formation of a 

 calcareous shell. This is often complex in its struc- 

 ture, and composed of several layers. It is usually 

 developed to such an extent that the weight of 

 mineral covering reduces the free movement of the 

 animal to a minimum. 



NeuralPlate 



.Epiderm 



>CutiS. 



FIG. 41. Diagrammatic transverse section through the shell of the 

 tortoise. On the right side the bony shields have been removed. The 

 endoskeleton is cross-lined, the exoskeleton is dotted. Sp.C, spinal 

 chord ; Cap, head of rib. (From Gadow.) 



Muscular System. The tissues involved in loco- 

 motion and other movements act by their power of 

 contraction. In order to bring about orderly move- 

 ments they must be attached to rigid supports. In 

 the vertebrates, these are the bones of the skeleton, 

 to which the muscles are firmly attached. The 

 bones themselves are covered with a tough skin of 



