470 



G. HERBERT FOWLER. 



tion, generally admitted to indicate an eight-mesenteried an- 

 cestor. Now this eight-mesenteried stage is unfortunately 

 reached in the Hexactinise by various methods. ^ The moment 

 that we begin to apply homologies to these octomeral larvse we 



The chief eight-rayed types of Hexactinian and Madreporarian larvse. The 

 numbers indicate the order of appearance of the mesenteries ; the dotted 

 outlines, the position of the future mesenteries which make up the total 

 twelve primaries. 



i. Fig. A represents Sagartia, Actinia, Buuodes (Lacaze-Duthiers, corrobor- 

 ated by F. Dixon), an unknown larva, ? Bunodes (Boveri), and Cereactis 

 (Boveri). 



ii. Fig. B holds for Adamsia (0. and R. Hertwig), and an unknown larva, 

 ? Tealia (Boveri). 



iii. A third type differs from that of Fig. A in transposing the order of appear- 

 ance of 2 and 4, and of 5 and 6 (not numbered in the figure) ; this occurs 

 in Rhodactis (McMurrich), Manicina (H. V. Wilson), and Cereactis 

 (Cerfontaine). 



iv. A fourth variation is described in Halcampa (Faurot), which transposes 

 5 and 6 of type iii. 



The four types therefore read — i. 3, 6, 1, 5, 4, 2; ii. 3, 1, 5 + 5, 2, 4; iii. 

 3, 5, 1, 6, 2, 4 ; iv. 3, 6, 1, 5, 2, 4. 



are met by a ring of diflB.cult choices. If the twelve primary 

 mesenteries are homologous throughout the various genera and 

 species, there are at least three lines of descent in the group, 



1 In the Cerianthese, according to van Beneden, the octomeral stage is 

 apparently attained by yet another path. 



