114 INVERTEBRATE ZOOLOGY. 



middle ventral region of the bodj" may be consolidated into a single plate to 

 offer a better support for the legs. Is the grasshopper a higher or lower niet- 

 americ animal ? In higher forms the segments may be grouped into three 

 body regions or segment complexes, the head, thorax and abdomen, each 

 eqtiipped with its peculiar parts, and fitted to perform a certain range of 

 functions. The head bears the sense organs, antennae, eyes, etc., and pos- 

 sesses internally the central organ of the nervous system, the brain. Its 

 function is therefore p.s^c/i«c. The thorax bears the legs and wings, and con- 

 tains the largest of the body-muscles for moving these parts. It thus controls 

 the animal functions. The abdomen bears the organs for egg-laying, fertiliz- 

 ing the eggs, etc., and contains the reproductive and digestive organs. These 

 functions are shared by plants and are termed vegetative. 



90. Morphology. Its methods. Morphology is the comparative study of 

 anatomical details. It considers every animal a plastic form, starting from a 

 simple primitive type, and moulded and modified to suit its peciiliar environ- 

 ments and conditions of life. It thus seeks to express every anatomical detail 

 in terms of the typical form, rather than bj' means which are purely arbi- 

 trary. This may be made plain by referring to the following j^ostulates, ap- 

 plicable to the group of animals under consideration. 



1. Typical somite.* The grasshopper is too much modified to obtain from 

 it a clear idea of a simple unmodified somite ; but the study of more primi- 

 tive forms yields the following conception : — An approximately circular ring, 

 composed mainly of two skeletal pieces, a larger, overlapping dorsal piece, 

 the tergite, and a smaller, ventral piece, the sternite. These are united by lat- 

 eral naembranes, in which a farther pair of pieces may develop, t\\e pleu rites 

 or pleura. This somite bears a pair of Jout/e^Z appendages, inserted between 

 the pleurite and sternite. 



2. Typii'al animal (of this group). Corresponding to the above definition, 

 a typical animal would be formed by a series of such somites, each overlap- 

 ping the next posterior. Each somite must be of the same size and shape and 

 must contain the same organs. There are no actual animals corresponding 

 exactly to this description, but the more primitive members of this group ap- 

 proach nearer to it. cf. Chilopoda and Peripatus. 



3. Modijications. The grasshopper may be considered as the result of a 

 series of gradxxally changing environments brought to liear upon the type 



* The above description is that of skeletal parts merely. 



