CHAPTER VIII 

 THE INVERTEBRATE BODY 



Nature is so varied in her manifestations that many must unite 

 their knowledge and efforts in order to comprehend her. — Laplace. 



From our survey of the Invertebrates it is obvious that the 

 highly complicated and varied organization of animals renders 

 it impossible to present a concise and adequate plan of a typical 

 animal body. It is therefore necessary in the present work to 

 select one group of animals as the basis of study and then to com- 

 pare with this, in so far as comparisons are possible without con- 

 fusion, a few of the most significant morphological and physio- 

 logical variations presented by other groups. We naturally select 

 the group of Vertebrates for chief consideration not only because 

 its relative homogeneity renders it the most available, but because 

 it includes Man. However, even before we focus attention on the 

 Vertebrates, it is necessary to bring into clear relief certain struc- 

 tural principles that we have seen exhibited among the Inverte- 

 brates — selecting as types the Hydra, Earthworm, and Cray- 

 fish — in order to afford a background for the consideration of 

 Vertebrate structure and function. 



A. Hydra 



In discussing the development of animals, it was pointed out 

 that the dividing egg typically forms first a blastula which, in 

 turn, becomes transformed into the gastrula stage. The gastrula 

 is essentially a sac composed of two layers of cells: an outer, or 

 ectoderm, and an inner, or endoderm, layer. Although no adult 

 animals retain this simple gastrula form, those composing the 

 group known as the Coelenterates are to all intents and purposes 

 permanent gastrulae since their bodies are built on the plan of a 

 two-layered sac. This is well exhibited in Hydra, the almost micro- 

 scopic, fresh-water polyp which is commonly found attached to 

 submerged vegetation or stones in brooks and ponds. (Fig. 31.) 



The body of Hydra somewhat resembles a long narrow sac, the 

 base constituting the foot, and the opening at the opposite end 



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