170 ACTINOZOA. 



seems to occur. Fertilisation is probably effected 

 while yet the ova remain within the somatic cavity 

 of the parent. Here, in many cases, the early 

 stages of development also take place. 



Section II. 



DEVELOPMENT OF ACTINOZOA. 



The life-history of the Actinozoa presents a 

 series of phenomena by no means so diversified as 

 those which have been shown to characterise the 

 developmental cycle in most of the Hydvozoa. 

 For here the embryo, by an easy and gradual suc- 

 cession of changes, tends finally to assume the 

 condition of an organism similar to that which 

 brought it forth. 



In the evolution of the embryo the whole or a 

 greater part of the fertilised ovum seems to be 

 concerned. The product of the reproductive act 

 usually soon appears as a ciliated body, while yolk- 

 cleavage, division into layers, and formation, by 

 liquefaction, of an internal nutrient cavity, take 

 place in the ordinary manner. 



ZoANTHARiA and Alcyonaria. — Among the 

 Zoantharia and Alcyonaria the further develop- 

 ment of the primitive polype into which the embryo 

 is resolved would seem to be, in most cases, as 

 follows. The young animal, still retaining its 

 cilia, assumes a somewhat oblong ovate form. A 

 central depression then makes its appearance at 

 one extremity, indicating the rudimentary mouth. 

 The internal cavity enlarges. Meanwhile, the 



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