430 



ZOOLOGY 



remain together to form a cooperative unit. In course 

 of time a cavity is formed within the developing organ- 

 ism or embryo, communicating with the outside by a 

 a single opening, called the blastopore. In a general 

 way this stage may be said to correspond with that of 

 the adult medusa or jellyfish, or with the sea anemone. 

 These lowly animals are shaped more or less like a bottle, 

 with a large cavity opening only at one end. It thus 

 The coeien- appears probable that our very remote ancestors passed 

 terate stage tnrou gh a ccelenterate stage, though of course this can- 

 not be demonstrated as a fact. The blastopore, which 

 is situated at what becomes the hind end of the body, 

 presently closes, and the permanent anterior and pos- 

 terior openings of the alimentary canal are formed by 

 new depressions or pits, which meet and become con- 

 tinuous with the ends of the central cavity. When this 

 has occurred, the embryo has reached what is in effect 

 a worm stage ; and although there is no close resem- 

 blance to any particular kind of worm, we can hardly 

 doubt that we have passed through a wormlike condi- 

 tion in the course of our evolution. 



All this may seem highly speculative, but from Our 

 knowledge of animal structure and development it ap- 

 pears impossible to imagine any other path of evolution 

 than the one suggested. Thus, for example, while a 

 non-scientific person might ask whether the first animal 

 was not after all wormlike, or fishlike, the so-called 

 lower forms resulting from degeneration and disinte- 

 gration, the biologist readily perceives so many difficul- 

 ties in the way of such a theory that he cannot even class 

 it among the possibilities. 



4. When we leave the worm stage, our principal diffi- 

 culties begin. From this point until the vertebrate 

 type is distinctly formed, the path of evolution is ob- 



Evolution 

 from a 

 wormlike 

 stage 



