No. 6.] THE MORPHOLOGY OF THE ACTINOZOA. 265 



ferent character, below which the characteristic epithelium of 

 the glandular streak comes into view. It seems to me from 

 these results that one is not justified in assuming, as has so 

 frequently been done, that the glandular streak epithelium is a 

 prolongation of the stomatodaeal ectoderm. What I have just 

 described, taken in conjunction with the observations of E. B. 

 Wilson ('84) on the development of the filaments in the Alcyo- 

 naria, and with what I have found ('91) as to their development 

 in the Hexactiniae, seems to me rather to point to a complete 

 distinction between the two kinds of epithelium, and I regard 

 the structure of the adult filament of Z. sociatiis as confirmatory 

 of the conclusions obtained from embryological studies, that the 

 ciliated bands of the filaments are ectodermal in origin, while 

 the glandular streak proper is an endodermal structure. 



III. The Development of the Filaments in Egg 

 Embryos. 



The material at my disposal for the study of the embryonic 

 development of the filaments was not sufificient for an exhaust- 

 ive study of the subject. The youngest larvae already pos- 

 sessed twelve mesenteries arranged in the manner described 

 by van Beneden ('90) and myself ('9ia). On none of the mes- 

 enteries were there any indications of the ciliated bands, but, 

 on the other hand, the glandular streaks were plainly indicated 

 on the perfect mesenteries as an epithelium occupying the free 

 edge of the mesentery and composed of cells with closely set, 

 elongated, and deeply staining nuclei, very different from those 

 of the general endoderm of the mesenteries. But what is more 

 interesting, on the lower part of the free edge of each of the 

 imperfect mesenteries a similar, but smaller, patch of epithelium 

 was plainly visible. In Fig. 10 is given a representation of a 

 part of a section through the lower portion of the column of one 

 of these youngest larvae. Owing to its base having been some- 

 what depressed by contraction, this has been cut towards the 

 central part of the section. Transverse sections of four mes- 

 enteries are shown ; the two larger mesenteries are one of the 

 macrodirectives (HI), and one of those which I have taken for 



