84 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



SECTION VI. 

 Nervous System of Invertebrates. 



The departments below the vertebiates we group 

 together under the general term invertebrates, not be- 

 cause they are more nearly related to one another than 

 they are to vertebrates, for this is not true, but because 

 we must treat them far more cursorily. 



i. Articulata. Arthropods and Annelids. — The plan 

 of structure of these is widely different from that of 

 vertebrates. In these (i) the skeleton is on the out- 

 side, the muscles acting from within to produce motion 



FlG. 55. — Diagram of section across an arthropod : b, blood system ; «, nerv- 

 ous system ; i, intestines. 



and locomotion ; instead of being within, and muscles 

 working on the outside for the same purpose, as in the 

 vertebrates. (2) Of all departments these animals are 

 the most distinctly and most numerously jointed, and 

 therefore they are called articulata. (3) The skeleton 

 being on the outside, it forms a hollow tube or cavity 

 inclosing all the viscera and organs of the body (Fig. 55), 



