PHYLUM CCELENTERATA 



193 



and one of the sixth pair ; a third couple is similarly formed by a mesentery of 

 the second and one of the fifth pair. Thus it is only in the case of the directive 

 mesenteries that an adult couple coincides with an embryonic pair : in other 

 instances the two mesenteries of a couple are of different orders, belonging 

 to distinct embryonic pairs. The mesenteric filameuts of the first cycle of 



Sid 



Fig. 142. — Trausverse sections of early (A) and later (B) stages of an embryo Sea-anemono 

 (Actinia.) The mesenteries are numbered in the order of their development ; std. stomo- 

 daeum. (After Korschelt and Heider.) 



mesenteries are parti}- ectodermal, partly endodermal in origin, those of the 

 remainder entirely endodermal. 



The tentacles are developed in a somewhat similar order to that of the 

 development of the mesenteries. The first to make its appearance is connected 

 with the larger or dorsal enteric chamber mentioned above : for some time it 

 remains much longer than any of its successors, and thus accentuates in a marked 

 degree the bilateral symmetry of the embryo. 



* 



It will be noticed that the development of the Sea-anemone is 

 accompanied by a well-marked metamorphosis, but that there is no 

 alternation of generations. In this respect its life-history offers a 

 marked contrast with that of Obelia. 



2. Distinctive Characters and Classification. 



The Actinozoa are Coelenterata which exist only in the polype- 

 form, no medusa-stage being known in any member of the. class. 

 The actinozoan. differs from the hydrozoan polype mainly in 

 possessing a stomodiEum : it differs from the hydrozoan and many 

 scyphozoan polypes in the possession of mesenteries or vertical 

 radiating partitions, which e.xtend inwards from the body- wall 

 and some of which join the stomodseum. The free margins of the 

 mesenteries bear coiled mesenteric filaments, which appear to 

 answer to the gastric filaments of Scyphozoa, but may be partly 

 ectodermal in origin. The mesenteries are developed in pairs, 



VOL. I o 



