OF COENOPSAMMIA FROM LIFU. 375 



They too are directly continuous with the stomodoeum and have the same structure, 

 so that fi-om the histology one must come to the conclusion that the whole filament of 

 the pnmary and secondary mesentenes is ectodermic in origin. It would appear also 

 most probable that the filaments of the tertiary mesenteries are likewise ectodemiic. 



I have already pointed out that the central glandular elements of the mesenterial 

 filaments have been shown to produce the' digestive secretion in both the Actiniaria 

 and in the Alcyonaria. In one polyp of Coenopsammia, which I have examined by 

 transverse sections, a small Crustacean is lying in the coelenteron, where it passes 

 into ■ the stomodoeum. It is noticeable that in spite of the strong contraction of the 

 polyp it is supported by the mesenterial filaments alone. Willem (29) too has shown 

 in several Actiuians that the prey is always clasped by the mesenterial filaments after 

 passing through the stomodoeum, and further has investigated the action of the 

 digestive secretion op proteids, glycogen and fats. Both Hickson for Alcyoniwm diyi- 

 tatum and Willem for various Actinians have brought forAvard negative evidence ti> 

 show that no particles of food are taken up in the solid form by the so-called 

 endoderm. Particles of carmine are however readily seized and the chief excretive 

 functions cippear to lie in this epithelium. There are no secretory digestive cells in 

 the so-called endoderm, and it follows hence that digestion must be brought about 

 by the ectoderm of the stomodoeum together with its downgrowths over the edges of 

 the mesenteries, forming their filaments. 



The stomodoeum of Zuantharia and necessarily also of Ailcyonaria is not comparable 

 then to the stomodoeum of the Triploblastica but rather is, with the mesenterial filaments, 

 the homologue of the ivhole gut. The so-called endoderm, giving rise to the muscular bunds 

 and generative organs and performing also the excretory functions, is then homologous tuith 

 the mesoderm of Triploblastica. In the terms of the layer theory, of whatever value it 

 may be, the Actinozoon polyp must then be regarded as also a Triploblastic form having 

 definite ectoderm, endoderm and mesoderm. 



Sedgwick (26) in 1884- pointed out the possible importance of considering the 

 Actinozoon polj'ps in connection with the origin of metameric segmentation in Triplo- 

 blastica, a view which was afterwards strongly supjjorted by van Beneden (2) from 

 his researches on the development of Arachnactis. The foregoing facts seem to me 

 to give a strong support to this hypothesis. It is however beyond the scope of this 

 paper to discuss either this question, or that of the relationship of the Actinozoon 

 and Hydrozoon polyps. 



51—2 



