ACTINOZOA. 



171 







testamentary system is gradually becoming dis- 

 tinct, thread-cells accumulating to form a super- 

 ficial layer, beneath which pigment globules may 

 also be observed. Soon the muscular substance 

 begins to be differentiated, its longitudinal fibres 

 and the first rudiments of the mesenteries being, 

 at an early period, discoverable. Next, small pro- 

 tuberances arise round the mouth, each of which 

 gradually elongates to form a tentacle. The initial 

 number of tentacles in the embryonic polype 

 always bears some relation to that observable 

 among the adult forms of the group of which it 

 is a member. Thus, in most Zoantharia either 

 five or six tentacles first sprout forth, but this 

 number is rapidly doubled, an increase in size of 

 the older tentacles being simultaneously effected. 

 But, in very young Alcyonaria eight tentacles 

 appear, as in the mature polype. 



Within the body-substance transverse contractile 

 tissues may now, at length, be detected. Minor 

 changes of external form also take place; the 

 cilia disappear, or are replaced by others of smaller 

 size; and the proximal extremity modifies itself 

 in accordance with the habits of the adult animal. 



But before the formation of its tentacles, the 

 young polype undergoes that important structural 

 change which distinguishes it from the rudimen- 



Hyd 



A circular fold of 



the body-substance surrounds the oral extremity 



manner 



the wide digestive sac, open above and below, and 

 freely communicating with the somatic cavity, 

 from which it, nevertheless, remains distinct. The 

 histological composition of the wall of this sac 



