198 



ZOOLOGY 



SECT. 



B 



-t 



have a very obviously bilateral arrangement : in this genus, as 

 growth proceeds, new mesenteries are added on the dorsal side, 

 and not, as is usual, between already formed couples. On the 



other hand, the genus Gyractis ex- 

 hibits a perfectly radial arrange- 

 ment : the mesenteries are all 

 arranged in couples with the longi- 

 tudinal muscles facing one another. 

 Lastly, in all the more typical 

 Sea-anemones (forming the tribe 

 Hexactinice) there are either six, 

 eight, or ten pairs of perfect 

 mesenteries, which, as well as the 

 secondary and tertiary cycles, are all 

 arranged in couples, the longitudinal 

 muscles of all but the one or two 

 directive couples facing one another. 

 In the Madreporaria the mesen- 

 teries are arranged, so far as is 

 FIG. 150 Edwardsia ciaparidii. known, in the way iust described 



A, the entire animal ; t. tube. B, . __ , *T 



transverse section. (After Andres, tor the HexactllUSe. In the Antl- 



patharia there are six primary and 



sometimes either four or six secondary mesenteries. In the whole 

 of the Alcyonaria the mesenteries are eight in number : they are 

 not arranged in couples, and their longitudinal muscles all face the 

 same way, viz., towards the ventral aspect (Fig. 149, B). In this 

 whole sub-class, therefore, the resemblance to Edwardsia is very 

 close, the main difference being that the longitudinal muscles 

 of the ventral directives face inwards in the Alcyonaria, outwards 

 in Edwardsia. 



The tentacles in Zoantharia are usually very numerous, and in 

 nearly all cases have the form of simple glove-fmger-like out- 

 pushings of the disc. In Edwardsia, however, they may be 

 reduced to six- 

 teen, and in some 

 genera of Sea- 

 anemones they are 

 branched. In the 

 Antipatharia (Fig. 

 151) they vary in 

 number from six 

 to twenty-four. When more than six are present, six of them 

 are larger than the others. 



In the Alcyonaria, on the other hand, the tentacles, like the 

 mesenteries, are eight in number and are always pinnate, i.e. 

 slightly flattened and with a row of small branchlets along 

 each edge (Fig. 145). Many Actiniaria have the tentacles 



FIG. 151. Antipathes ternatensis, portion of a branch, 

 showing three zooids and the homy axis beset with spines. 

 (From the Cambridge Natural History, after Schultze.) 



