NERVOUS SYSTEM OP INVERTEBRATES. 



8 9 



the wings and legs, as well as to preside over the stomach, 

 etc. (Fig. 62). It is well to observe that the consolidation 



Fig. 62. — Diagram showing- the embryonic development of a lepidopter : 

 a, caterpillar ; d, chrysalis ; c, the perfect butterfly. 



of the segments of the skeleton follows the same course 

 — goes hand in hand with that of the nervous system. 



2. MollllSCa. Comparison with Other Departments, — 

 The whole plan of structure of these is again different. 

 The vertebrates and arthropods both have a true loco- 

 motive skeleton, the one interior, the other exterior. In 

 both, also, but especially the latter, the skeleton con- 

 sists of segments repeated in a linear series. The mol- 

 lusks have no locomotive skeleton, but only a pro- 

 tective shell. The mollusks also have no segmented 



