60 HYDR^, JELLYFISHES, SEA ANEMONES, CORAL POLYPS 



tentacles, live in small holes in this layer of flesh, at right 

 angles to the red stem within. When undisturbed, they 

 project beyond the opening with tentacles spread out to 

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

 for protection. 



Sea Walnuts 



Glass IV. — Ctenophora {comb bearers) 



These are peculiar jellyfishes, mostly small, 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 polyp form as in other Coelenterata we have studied. 

 These organisms are extremely delicate and are usually 

 perfectly transparent. They swim by means of ciha and 

 many of them are highly phosphorescent. 



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 aureUa, and other coelen- 

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

 that a generation of each of two dissimilar individuals 

 — the plantUke form and the medusa — was necessary 

 to complete the full life history of the campanularian 

 hydroid. Moreover, these generations followed each other 

 alternately; the medusa giving rise to the planthke struc- 

 ture, and the planthke structure giving rise 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 imder more than two forms are said to 

 be polymorphic. 



