IV 



PHYLUM OELENTERATA 



191 



teries (2) appear in the dorsal chamber, dividing it into a median and two lateral 

 compartments ; then a third pair (3) in the ventral chamber, producing a 

 similar division ; then a fourth pair (4) in the middle compartment of the 

 dorsal chamber ; then a fifth pair (B, 5) in the lateral compartments of the 

 dorsal chamber ; and a sixth (6) in the lateral compartments of the ventral 

 chamber. Soon the longitudinal muscles are developed, and the fate of these 

 primitive pairs of mesenteries can be seen. The third and fourth pairs become 

 the two directive couples of the adult ; another couple of primary mesenteries 

 is constituted, on each side of the vertical plane, by one of the mesenteries of 

 the first 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 filaments of the first 



std 



B 



FIG. 143. Transverse sections of early (A) and later (B) stages of an embryo Sea-anemone 

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

 daeum. (After Korschelt and Heider.) 



cycle of mesenteries are partly 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 Coslenterata 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 : it differs from the hydrozoan and many 

 scyphozoan polypes in the possession of mesenteries or vertical 

 radiating partitions, which extend inwards from the body-wall 



