CHESTER. — STRUCTURE OF PSEUDOPLEXAURA CRASSA. 759 



are quite abundantly scattered in the endodenn of the coelentcra, 

 inc'hidinu- the canals and the mesenteries, may considerably aid in 

 extra-cellular digestion. 



Dorsal Mesenterial Filaments. 



These filaincnts differ in origin, structure, and use from the ventral 

 and lateral filaments. As a whole, the filament is a long, deeply 

 grooved ribbon or cord, attachetl to the nuirgin of the corresponding 

 mesentery, and reaches from the stomodaeum to the depths of the 

 polyp cavity; if there is a large basal canal connecting polyps with one 

 another, it may even be continued into such canals. In cross section 

 (Plate 4, Figs. 62, 63) the filament is much thicker than the mesentery 

 and is deeply notched at its free margin. Consequently, in cross 

 sections the mesogloea has the form of the letter Y. The epithelial 

 cells occupying the space between the arms of the Y are of two kinds. 

 The outer ones (c/. fil.), those nearer the ends of the arms of the Y, 

 are the more numerous and are similar to the supporting cells of the 

 stomodaeum. They are columnar, of small diameter and so closely 

 packed that their nuclei are arranged in several roMs. Each cell has 

 a very strong cilium, and these cilia are so long that those of one side 

 of the groove touch or cross those of the opposite side. The remain- 

 ing cells, those occupying the base of the filament groove (c/. fil.c), 

 are few but larger, having broad bases and tapering slender necks. 

 Their cell boundaries usually cannot be demonstrated. Near the 

 base of each cell is a large, lightly staining nucleus. They possess no 

 cilia. The cytoplasm is sometimes evenly granular, but often shows 

 large vacuoles that stain with muci-carmine. The mucus, which they 

 evidently have secreted, may sometimes be found between the cilia 

 of the other cells. 



^^'ilson ('84) has described in detail these dorsal filaments for other 

 alcyonarians. But neither in his eleven genera, nor in the figures of 

 Alcyonium by Hickson ('95), nor of Xenia by Ashworth ('99), are the 

 cells represented to be as large and prominent as they are in Pseudo- 

 plexaura. I did not observe the effect of the presence of mucus in 

 this groo^■e, sa\e that sometimes very minute particles, presumably 

 of food, may be found in it; the mucus is probably for the purpose of 

 catching material entering the polyp from another polyp or from the 

 long canals. Portions of colonies kept for some days in the dark or 

 in weak light lost their Zooxanthellae. The polyps of these portions 



