104 ANIMAL BIOLOGY 



which divides the body into right and left sides passes through the 

 upper (dorsal) and lower (ventral) side : the body exhibits bilat- 

 eral symmetry which is characteristic of higher animals. (Fig. 61.) 



2. Tissues and Organs 



Bilateral symmetry practically implies the existence of definitive 

 parts of the body, or organs and organ systems, and these we find 

 highly developed in the Earthworm. Again, the presence of organs 

 demands a much greater differentiation of tissues than occurs in 

 Hydra where local modifications of ectoderm and endoderm serve 

 the purposes of its relatively simple organization. Accordingly in 

 the Earthworm and in all higher forms the mesoderm is added to 

 the two primary cell layers, and from these three there is developed 

 a great variety of special tissues: epithelial, supporting, muscular, 

 circulating, nervous, and germinal. Finally, the cooperation of 

 tissues to form organs demands the further cooperation of organs 

 to form organ systems, each of which plays its part in the economy 

 of the whole organism. (Figs. 32, 34.) 



Thus it is clear that the body plan of the Earthworm and all 

 higher forms is radically different from that of Hydra, exhibiting 

 as it does such essential new features as mesoderm, coelom, bilateral 

 symmetry, segmentation, specialized tissues, definitive organs, and 

 complex organ systems. The persistence and development of this 

 basic plan from Earthworm to Man is interpreted by biologists as 

 evidence of evolution. 



C. Crayfish 



Bearing in mind the general plan of the body of the Earthworm, 

 we must next consider briefly the main principle underlying the 



Cerebral ganglion Intestine Dorsanjlood^^.^^ exoskelet on 



Mouth 

 appendages 



Subesophageal Ventral nerve Jointed 



ganglion cord ganglion appendages 



Fig. 62. — Diagrammatic representation of the structure of an ideal primi- 

 tive Arthropod in which very little specialization of the segments has occurred. 

 (From Schmeil.) 



changes in this plan which give rise to many of the diverse forms 

 among the higher animals. 



