GENERAL SURVEY. 129 



Portuguese man-of-war, which show, on a different plane, 

 as much division of labour as Hydractinia. 



The same general conclusions apply to the jelly-fish and 

 sea-anemones. The jelly-fish present a strong resemblance 

 to the medusoids, but are distinguished 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. But even 

 here we find one type (Pelagia) always locomotor, another 

 {Aurelia) whose early life is sedentary, and others (Lu- 

 cernarians) 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 

 swimming-bells (Fig. 55). They are, however, much more 

 complicated in structure than the hydroids. Solitary 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 commonly 

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

 results in the formation of corals. It should be noted, 

 however, that various quite distinct polypoid types may 

 form corals. Thus, while the most important reef-building 

 corals are included in the Scyphozoa, the Millepores among 

 the Hydrozoa also form very considerable calcareous colonies. 



Finally, as the corals are predominantly passive, so there 

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

 cilia united into combs, and often shine with that " phos- 

 phorescence " which is an expression of intensity of life in 

 many active animals. The origin of the Ctenophores is 

 still obscure ; Gcette believes that they have arisen from 

 a scyphula, the hypothetical ancestor of the Scyphozoa. 



As to diet, the active Ctenophores are carnivorous, 

 attaching themselves by adhesive cells to one another, or 

 to other small animals ; many of the larger forms, e.g. sea- 

 anemones and jelly-fish, are able to engulf booty of 

 considerable size ; the majority, however, feed on small 

 organisms, in seizing and killing which the tentacles and 

 stinging cells are actively used. 



