86 



INVERTEBRATA 



CHAP. 



have been formed, and then the twelve are developed in a single 

 cycle. The twelve mesenteries are arranged in six pairs which 



correspond to the six pairs of Hexactiniae, 

 but the dorsal "directives" are short and 

 do not reach the stomodaeum, and one 

 mesentery of each of the lateral and ventro- 

 lateral pairs remains short, whilst its fellow 

 is long and joins the stomodaeum. In the 

 space which intervenes between the ventral 

 "directives" and the ventro- lateral pair 

 on each side, there is a growing zone where 

 new pairs of mesenteries are added, one 

 fellow of each new pair being long and one 

 short. 



It appears from this brief review that 

 the larvae of Cereanthidae, unlike the 

 Hexactiniae, do not pass through an "Edwardsia" stage in their 

 development, but that nevertheless they may be regarded as springing 

 from a stock, common to the Hexactiniae, Edwardsiae, and themselves, 



FIG. 65. The Zoauthhia larva 

 of a Zoanthid. (After 

 Carlgren.) 



A 



stem 



FIG. 66. Transverse sections of Actinozoan larvae. (After van Beneden.) 



A, section of Zoanthina larva. B, section of Arachnactis larva, d, (in A) dorsal directive 

 mesenteries, (in B) indefinite number of mesenteries occupying the place of the dorsal directives ; 

 <2.2, dorso-lateral mesenteries ; rt.P, long dorso-lateral mesenteries ; d.l 2 , short dorso-lateral mesenteries ; 

 v, ventral directive mesenteries ; v.l, ventro-lateral mesenteries ; v.l 1 , long ventro- lateral mesenteries ; 

 v.l' 2 , short ventro-lateral mesenteries ; xx (in A) mark the places where additional pairs of mesenteries 

 are added. 



in which only six mesenteries were developed. The Zoanthidae on 

 the contrary may almost be said to pass through a Hexactinian 

 stage in their development. 



We have much less information on the development of Alcyonaria 

 than on that of Zoantharia. The best account is that of Wilson, to 

 which the more recent accounts by Koch of the development of 

 Gorgonia (1887), and by Hickson of the development of Alcyonium 



