McMURRICH. 



[VOL. II. 



FIG. 2. Diagram showing the arrangement of the mesenteries 

 and tentacles in Halcurias pilatus. 



exocoel is much narrower than either of the adjacent endocoels 

 and contains no imperfect mesenteries, while in the next endo- 

 coel two pairs of mesenteries are seen on the right side and 

 one on the left. By following through the series of sections 

 it is readily seen that the arrangement found in the first lateral 

 endpcoel of the right side is repeated in all the others, except 

 in the cases of the endocoels enclosed by the directives, and 



the condition repre- 

 sented diagrammat- 

 ically in Fig. 2 is 

 that which obtains. 

 Owing to the rel- 

 ative widths of the 

 endocoels and exo- 

 coels, and the pres- 

 ence of imperfect 

 mesenteries in the 

 former, the first impression one receives is that of a form with 

 a large number of directive mesenteries. That such an inter- 

 pretation of the conditions is erroneous is clearly shown, how- 

 ever, by reference to the mesenteries on either side of the true 

 directives. It is interesting to note that the development of 

 the imperfect mesenteries, which are plainly arranged in two 

 cycles, follows the hexactinian rule, the smaller pairs being 

 developed in the intervals between the larger pairs and the 

 adjacent perfect mesenteries. It may be added that my sec- 

 tions show the existence of a marginal stoma in each perfect 

 mesentery in addition to the oral stoma already mentioned. 



From the description given above, it will be perceived that 

 the arrangement of the mesenteries in Halcurias pilatus is 

 identical with that described by Carlgren ('97) for a form 

 from the Chinese seas which he refers to the genus Endo- 

 coelactis and to a new family, the Endocoelactidae. The simi- 

 larity to Halcurias is by no means confined, however, to the 

 arrangement of the mesenteries, and there can be no question 

 but that the two forms must be referred to the same genus, 

 to which, notwithstanding the greater appropriateness of Carl- 

 gren's name, the prior term, Halcurias, must be applied. The 



