CHESTER. — STRUCTURE OF PSEUDOPLEXAURA CRASSA. 709 



separate elsewhere. In the tentacles, in the polyp wall and in many 

 canals they contain large numbers of the alga Zooxanthella. Except 

 in the tentachvs and tlie longitudinal canals, they have myonemes 

 running circularly. Unicellular nuicus glands and granular cells, that 

 are probably digestive in function, are numerous. The cells of the 

 longitudinal canals differ from other endodermal cells in being much 

 longer and in liaA'ing no trace of myonemes. 



Digestion is accomplished by cells of the stomodaeum, by the six 

 ventral and lateral mesenterial filaments, and by scattered gland cells 

 in the walls of the pol\'p cavity and the canals. The stomodaeum has, 

 beside its supporting cells, mucus and granular gland cells. The 

 mesenterial filaments, except the dorsal pair, are very short and their 

 epithelium is composed of granidar gland cells only, which give some 

 evidence of intracellular digestion. I found no special feeding time 

 antl no regular alternation of contraction and expansion of polyps. 

 Slow-moving organisms, which serve as food, are often transferred 

 from the surface of the coenosarc between the polyps, where large 

 nettle cells abound, to the mouth of a polyp that independently con- 

 tracted to the le^■el of the coenosarc with its mouth open. The two 

 dorsal mesenterial filaments are very long and sinuous and their cell 

 structure is peculiarly significant. The sides of the groove are lined 

 by cells with strong cilia. The central cells, however, show the char- 

 acter of mucus cells and produce a mucous secretion. 



The muscle system is similar to that of Alcyonium. The colony 

 is characterized by the weakness of its responses and b\' the fewness 

 of its nerve elements. The response to touch is not quick, and the 

 coenosarc between the polyps is more sensitive than the polyps them- 

 selves. 



The axis skeleton is surrounded by an epithelium consisting of 

 elongated secreting cells, and in places, of desmocytes, or holding cells, 

 these being shorter and wider, and exhibiting striations at the axial 

 end. These cells become connected with the mesogloea secondarily. 

 They may become isolated as the result of being completely enveloped 

 in the secretion of homy material by the secreting cells. The desmo- 

 cytes have already been described for Heliopora and for the madre- 

 porarians. The evidence in Pseudoplexaura favors an ectodeimal 

 origin of the axis skeleton. 



