No. i.] MORPHOLOGY OF THE ACTINOZOA. 129 



tives, but by a retardation of their formation. The fourth 

 formed mesenteries of Bunodes verrucosus are equivalent mor- 

 phologically to the second pair of Rhodactis, but their appear- 

 ance is delayed until after the formation of the directives. The 

 development of their mesenterial filaments before those of the 

 directives is thus explicable. 



A possible explanation of the earlier development of the fourth 

 pair is suggested by their relation to the dorsal siphonoglyphe. 

 It may be that the development of a second siphonoglyphe in 

 the Hexactinke has made it of advantage to the animals that the 

 mesenteries in connection with it should be early developed. It 

 may be noted in this connection that in RJiodactis, in which in 

 the adult two well-marked siphonoglyphes are present, the origi- 

 nal arrangement obtains, and at the same time the development 

 of the siphonoglyphes is not evident in the larva until the forma- 

 tion of the third and fourth pairs 

 of mesenteries. In other words, 

 the siphonoglyphes and the direct- 

 ives appear simultaneously. If an 

 early development of the dorsal 

 siphonoglyphe be of advantage to 

 the embryo, there would naturally 

 be an early development of the 

 dorsal directives. 



As regards the formation of 

 the fifth and sixth mesenteries, ~~1JL 



Which convert the embryo f rom , FlG - \- ~ Transve rse section through 



J larva of Adamsia (after Hertwig). 



the Edwardsia stage to the Hal- 



campa stage, Lacaze-Duthiers found that they developed be- 

 tween the ventral directives and the ventro-lateral mesenteries, 

 and between the ventro- and dorso-laterals. This observation 

 has been abundantly corroborated, and there can be no question 

 as to its correctness. The Hertwigs ('79), however, found a 

 different arrangement in Adamsia ; here the fifth and sixth 

 mesenteries form in pairs on each side between the dorso- and 

 ventrolaterals (Fig. I.). This arrangement has likewise been 

 confirmed by Boveri. 



We have consequently two different methods for the arrange- 

 ment of the fifth and sixth pairs of mesenteries, showing again 



