PHYLUM CCELENTERATA 169 



glance one generation is quite different in structure from 

 the other, but if some details are ignored, striking simi- 

 larities between polyp and medusa are apparent. Fig. 68 

 shows that a simple polyp may readily be transformed to 

 a medusa by two changes an additional ring of tentacles 

 must grow out at x t and the part of the polyp bearing the 

 mouth and tentacles will have to be pushed back into a cup 

 formed by an outgrowth at the bases of the new set of 

 tentacles. 



If we compare Obelia with the sponge described in the last 

 chapter, it is at once apparent that we are dealing with 

 a more progressive animal. The sponge excels Obelia 

 only in its great elaboration of tissues. Though a hydroid 

 has its tissues rather poorly defined, it shows progress in 

 other ways: there is a definite digestive cavity; the ten- 

 tacles are organs, for they consist of different tissues which 

 cooperate to capture food and poke it into the enteron. 

 There is coordination between different parts of the body 

 in a hydroid, whereas a sponge must have each of its tis- 

 sues activated by a direct stimulus. If meat juice is 

 squirted in the water near an Obelia, the tentacles wave 

 toward the source of the extract and the mouth is also 

 bent in the same direction. If the tip of a tentacle is in- 

 jured the whole hydranth withdraws within the protection 

 of the peritheca. A sponge is a rather loosely coordinated 

 federation of tissues; a hydroid is a well-organized metazoan . 

 with a definite digestive cavity and simple organs. 



Obelia may be looked upon as an average hydroid. Other 

 representatives vary greatly in the degree to which poly- 

 morphism and metagenesis have been developed. The 

 fresh-water Hydra (Fig. 68, Polyp) shows a very simple 

 condition. It consists of a simple polyp which may at times 

 form new individuals by budding and at other times give 

 off eggs or spermatozoa. The zygotes grow into polyps and 

 there is thus no metagenesis and no polymorphism. The 

 Portuguese man-of-war, Physalia (Fig. 70), which floats 

 in tropical seas, shows an extreme case of polymorphism. 



