No. 3.] MORPHOLOGY OF THE ACTINOZOA. 327 



to pass, without any solution of continuity, directly into the 

 stomatodaeal ectoderm. It would seem, then, that the lateral 

 streaks of the filaments are ectodermal in their origin, and 

 consist of two down-growths of the stomatodoeum along the 

 sides of the mesenteries close to their free edge. 



The third or median process of this portion of the filament 

 has a quite different structure. It does not at all resemble the 

 lateral streaks of the filament, neither is it similar to the median 

 streak (Nesseldriisenstreif), which is found lower down. It 

 appears to be more or less undifferentiated, and resembles the 

 endoderm in its structure rather than the ectoderm, although 

 the similarity is rather obscured by the presence in the general 

 endoderm of large numbers of Zooxanthellce, which are absent 

 in the median process. I believe it to be endodermal in its 

 nature, and to be the persisting endoderm which covered the 

 free edge of the filament before the lateral streaks made their 

 appearance, the endoderm at the sides of the free edge only 

 being pushed down or replaced by the ectodermal Flimmer- 

 streifen. 



I have not considered it necessary to review the literature 

 upon the formation of the mesenterial filaments ; this has been 

 sufficiently done in the papers of E. B. Wilson and H. V. Wil- 

 son, already referred to. It may be seen that my results as 

 stated above agree perfectly with those obtained by E. B. 

 Wilson ('84), and fully bear out the suggestion advanced by 

 him that the Flimmerstreifen of the Hexactinian filament is 

 equivalent to the dorsal mesenteries of the Alcyonaria, and that 

 the median streak (Nesseldriisenstreif) is equivalent to the six 

 ventral filaments of members of that group. On the other 

 hand, I am utterly at variance with H. V. Wilson on the ques- 

 tion of the origin of both portions of the filament. Wilson 

 homologizes the simple unlobed filament of Mayticina with the 

 entire trilobed filament of such a form as Aulactinia. He sup- 

 poses that in primitive forms the filament was simple and 

 unlobed, and that by a process of physiological differentiation 

 became divided into a central glandular and lateral respiratory 

 portions. This theory I cannot accept. The mere fact that I 

 believe in the origin of the two portions of the filament from 

 different germ-layers precludes its acceptance ; but in addition 

 the manner of formation of the lateral streaks shows them to 



