00 I1YDR7K, JKLLVFISIIEfi, SEA ANEMONES, CORAL TOLYPS 



tentacles, live in sinall h(jles in this layer of flesh, at right 

 angles t(j the red stem within. When vmdisturbed, they 

 project beyond the opening with tentacles spread out to 

 catch food; but when irritated, they withdraw into the hole 

 for protection. 



Sea Walnuts 



Olass IV. — C'lenophom {comb bearers) 



These are peculiar jellyhshes, mostly sinall, which spend 

 a free-swimming life and, so far as is known, never form 

 colonies. Neither is there at any stage of their existence 

 a pol}'p form as in other Cadenterata «'e liave studied. 

 These organisms are extremely delicate and are usually 

 perfectly transparent. They swim by means of cilia and 

 many of them are highly phosphoi'escent. 



Alternation of generations among the Coelenterata. — Like 

 many of the lower plants — fungi, mosses, and ferns — 

 many of the lower animals show what is known as an 

 alternation of generations. This phenomenon is shown by 

 the campanularian hydroid, the aurelia, and other co?len- 

 terates of which we have not spoken. It will be recalled 

 that a generation of each of two dissimilar individuals 

 — the plantlike form and the medusa — was necessary 

 to complete the full life history of the campanularian 

 hydroid. Moreover, these genei'ations followed each other 

 alternately; the medusa giving rise to the plantlike struc- 

 ture, and the plantlike structure giving lise to the 

 medusa, and so on indefinitely. Again, the campanularian 

 hydroid may be said to exist under two different forms. 

 This is called dimorphism (di , two; morphe, form). Some 

 animals that exist under more than two forms are said to 

 be polymorphic. 



