Volume I. October; 1897. Number J. 



ZOOLOGICAL BULLETIN. 



CONTRIBUTIONS ON THE MORPHOLOGY OF 

 THE ACTINOZOA. 



IV. ON SOME IRREGULARITIES IN THE NUMBER 

 OF THE DIRECTIVE MESENTERIES IN THE 

 HEXACTINL^. 1 



J. PLAYFAIR McMURRICH. 



THE discovery of three of the principal features of the mor- 

 phology of the Hexactiniae, - -the hexamerous arrangement of 

 the mesenteries, their association in pairs, and the occurrence 

 of directives, --we owe to several observers. Ehrenberg ('34) 

 was, I believe, the first to note the hexamerous arrangement; 

 the association of the mesenteries in pairs was first discovered 

 by Ercll ('42) but fully worked out by Hollard ('51) a few years 

 later; while the occurrence of directives was first observed by 

 Thorell ('58), and more fully described by Schneider and Rotte- 

 ken in 1871. The study of numerous species has demonstrated 

 the general occurrence of these three characteristics, which may 

 be regarded as normal for the group, but departures from the 

 normal arrangement have also been observed. Thus, instead 

 of hexamerism, a decamerous or an octamerous symmetry may 

 obtain ; indeed, one of the earliest thorough studies of mem- 

 bers of the group, namely that by Hollard, was made upon a 

 decamerous form. So too, although two pairs of directive 



1 Nos. I-III of these Contributions were published in the Joiirnal of Mor- 

 phology, vols. iv and v. 



