200 



ZOOLOGY 



SECT. 



B 



is the arrangement of the mesenteries. In Edwardsia (Fig. 149), 

 a genus which burrows in sand instead of attaching itself to 

 rocks, &c., there are only eight mesenteries (B) the usual two 

 couples of directives, and two others on each side of the vertical 

 plane, having their longitudinal muscles directed ventrally, and 

 therefore not arranged in couples. The adult Edwardsia thus 

 corresponds with a temporary stage in the development of one of 

 the more typical sea-anemones, viz., the stage with eight mesen- 

 teries shown in Fig. 142, A. ; it is probably to be looked upon as 

 the most primitive or generalised member of the order. In 



Zoanthus (Fig. 143, B) the dorsal 

 directives (d.d.) do not reach the 

 gullet, and each lateral couple con- 

 sists of one perfect and one small 

 and imperfect mesentery. In Ceri- 

 anthus, another burrowing form, 

 there is a couple of very small 

 ventral directives, and the remain- 

 ing mesenteries are very numerous, 

 not arranged in couples, and all 

 directed ventrally at their outer 

 ends, so as to have a very obviously 

 bilateral arrangement : in this genus, 

 as growth proceeds, new mesen- 

 teries are added on the dorsal side, 

 and not, as is usual, between already 

 formed couples. On the other hand, 

 the newly discovered Gyractis ex- 

 hibits a perfectly radial arrange- 

 ment : the mesenteries are all 



arranged in couples with the longitudinal muscles facing one 

 another. Lastly, in all the more typical Sea-anemones (forming 

 the tribe Hexactinioe) 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 mesenteries are arranged, so far as is 

 known, in the way just described for the HexactiniaB. In the 

 Antipatharia 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. 148, 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 lace inwards in the Alcyonaria, 

 outwards in Edwardsia. 



FIG. 149 Edwardsia claparedii. 



A, the entire animal ; t. tube. B. 

 transverse section. (After Andres, 

 and Korschelt and Heider.) 



