134 



FOUNDATIONS OF BIOLOGY 



ganglionic masses. The brain of the Crayfish, for example, 

 comprises the primitive ganglia of the segments which have 

 coalesced to form the head. (Fig. 73.) 



FIG. 73. Diagram of the general plan of the anterior portion of the central 

 nervous system of an Earthworm and a Crayfish, a, brain (cerebral, or supraoesopha- 

 geal, ganglion); 6, nerve commissures, encircling the pharynx (shown in section); c, 

 suboesophageal ganglion; d, ganglia of the ventral nerve cord, with nerves emerging. 



We have now considered the fundamental body plan of 

 Hydra, Earthworm, and Crayfish. These Invertebrate types 

 afford an excellent background for a proper understanding 

 of the body structure of the Vertebrate groups. Hydra 

 exhibits the simple two-layered condition (ectoderm and 

 endoderm) which is a transient phase in the early develop- 



