DEVELOPMENT OF EGGS IN MANY-CELLED ANIMALS 339 



blastula to ancestral honours, the present writer would point out 

 that all of them can be more or less completely matched by 

 members of the group of Protozoa, though the placula comes off 

 worst in this respect. And the possibility is by no means excluded 

 that different groups of many-celled animals have arisen indepen- 

 dently from colonial Protozoa which did not all conform to the 

 same type. 



A brief sketch of the chief stages in the development of a 

 few typical animals will form a fitting conclusion to the present 

 section. 



Development of the Freshwater Polype 

 {Hydra). — With the possible exception of 

 Sponges (Porifera), the great group of the 

 animal kingdom (Ccelenterata) of which Hydra 

 is a member may be regarded as the lowest 

 division of many-celled forms (Metazoa). Di- 

 gestive arrangements are of such great impor- 

 tance to the well-being of animals that we 

 might expect them to be evolved very early 

 in the history of the Metazoa. And it has 

 been remarked with truth that Coelenterates 

 may be epigrammatically defined as "living 

 stomachs ". This is certainly the case in 

 Hydra, the body of which is a hollow cylin- 

 der, with a mouth at one end, surrounded by 

 a circlet of tentacles that catch food, which is 

 taken into the large central cavity and there 

 digested. The numerous cells making up the body are disposed 

 in two layers, and this may be taken to represent commencing 

 division of physiological labour. The inner layer {endoderni), 

 lining the central cavity, is concerned with digestion, and the 

 outer layer {ectoderm) is not only protective, but also performs 

 the duties which are carried out in higher forms by the nervous 

 system and sense organs. In Hydra there is a thin structure- 

 less supporting membrane {mesogloed) between the two cell-layers, 

 and in many Coelenterates, such as jelly-fishes, this may be ex- 

 tremely well developed, and contain various cells which have 

 wandered into it from ectoderm and endoderm. But it is not a 

 continuous sheet of closely-connected cells, as are the two latter, 

 and therefore all Coelenterates are regarded as being two-layered 



Fig. S60. — Diagrammatic 

 Longitudinal Section of Fre-sh- 

 water Polype ( Hydj-a], enlarged 



jn. Mouth; y", foot; d, diges- 

 tive cavity; ec, ectoderm; en, en- 

 doderm ; inc, mesoglf^a; h. bud ; 

 00, ovaries ; ss, spermarie.'^. 



