GENERAL SURVEY OF CCELENTERA. 133 



Portugese man-of-war, which show, on a different plane, as 

 much division of labour as Hydraclinia. 



All these types belong to the class Hydrozoa, but the 

 same general conclusions apply to the next class, that of 

 the sea anemone and jelly fish. The jelly fish present a 

 strong general resemblance to the medusoids, but are 

 separated from them by their usually greater size, as well as 

 by greater complexity and distinct anatomical differences. 

 It is in accordance with this increased complexity that the 

 alternation of active and passive forms, though as real, is 

 less obvious. Yet even here we find one type {Pelagia) 

 always locomotor, another {Aurelia) whose early life is 

 sedentary, and others (Lucernarians) which in their adult 

 life are predominantly passive, and attach themselves by a 

 stalk. 



The sea anemones and their numerous allies may be 

 regarded as bearing a relation to the jelly fish, somewhat 

 similar to that which the hydroid polypes bear to the swim- 

 ming bells (Fig. 38). They are, however, much more com- 

 plicated in structure than the hydroids. Simple forms are 

 much commoner than in the Hydrozoa, but the colonial 

 type is nevertheless very frequent. The colonies may be 

 supported by an organic framework only, but very com- 

 monly there is a tendency to accumulate lime in the tissues, 

 which results in the formation of corals. 



It may be well to note here explicitly that various polypoid types 

 may form corals. In fact the formation of a framework of carbonate of 

 lime may be regarded as an expression of the sedentary constitution. 

 The most important reef building corals are included in the Scyphozoa, 

 but among the Hydrozoa the Millepores form very considerable lime 

 sheathed colonies. 



The corals present many problems of great interest. How do they 

 get their carbonate of lime ? Is that salt particularly abundant about 

 the reefs, or is there, as Irvine and Murray suggest, an interaction 

 between the waste products of the polypes and the sulphate of lime 

 abundant in sea water? On what do they feed? Do their bright pig- 

 ments, as Hickson suggests, enable them to utilise carbonic acid after 

 the manner of plants ? We may think also of the struggle for standing- 

 room among the coral polypes, and of the struggle for existence among 

 the many brightly coloured animals which browse and hide on the 

 reef 



Finally, as the corals are predominantly passive, so there 

 is a climax of activity in the Ctenophores, which move by 



