180 ZOOLOGY 



SECT. 



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 



Std 



FIG. 129. Transverse sections of early (A) and later (B) stages of an erpbryo Sea-anemone 

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

 dseum. (After Korschelt and Heider.) 



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

 to distinct embryonic pairs. 



The tentacles are developed in a somewhat similar order. 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 or of Aurelia. 



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 stomodaeum the relative size and physiological 

 importance of which are far greater than in the Scyphozoa, the 

 first group in which this structure is 'met with: it differs from 

 both hydrozoan and scyphozoan polypes in the possession of 

 mesenteries or vertical radiating partitions, which extend inwards 

 from the body-wall, and in some cases join the stomodaeum. 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 



