756 PROCEEDINGS OF TJHE AMERICAN ACADEMY. 



:08), and is not greatly different in its cell characters from Xenia 

 (Ashworth, '99) or Veretillum (Buvor, :01). The nematocysts of the 

 larger kind are about the size of those of Clavularia (von Koch, '82*), 

 and are generally shghtly larger than those of Xenia or Alcyonium. 

 Those of the smaller kind seem significantly numerous in the stomo- 

 daeum. Sense cells like those of Alcyonium (Kassianow, :08) are 

 associated with the nettle batteries; but the number of the l)atteries 

 in the coenosarc and at the base of the polyps appears to be greater 

 than in Xenia or Alcj'onium, and the fewness of the nematocysts on 

 the tentacles and pinnules seems unusual. Evidently the surface of 

 the coenosarc between the polyps is an important region for the work 

 that the large nematocysts do. The polyp often contracts to the level 

 of the coenosarc with the mouth still open and the tentacles still 

 spread, and I have seen food particles passed along from the coenosarc 

 to the mouth by the tentacles. The nematocysts of the smaller sort 

 are more evenly distributed on the outside of the colony, but they are 

 very few. I failed to find glaiid cells on the outer surface, except per- 

 haps in the coenosarc. Von Koch ('87) does not include them in his 

 list of ectoderm cells for Gorgonacea, but they are present in repre- 

 sentatives of the two other alcyonarian groups. It seems hardly 

 probable that the slime which is given off by Pseudoplexaura when it 

 is handled has come from the mucus cells of the endoderm. The 

 fibers of certain of the ectoderm cells of the coenosarc (Plate 3, Fig. 24) 

 are, as has already (p. 750) been suggested, possibly supporting 

 fibers, such as K. C. Schneider (:02, p. 622, Fig. 510) has de- 

 scribed for Anthozoa and other invertebrates, and figured for a sea 

 anemone. 



The mesogloea of the colony is very thin except in the coenosarc 

 region ; but here is thicker than that of the forms heretofore described, 

 except the Gorgonacea. 



The endoderm is similar to that of Alcyonium, and shows no signi- 

 ficant features, except the absence of muscle fibers in the longitudinal 

 canals. Menneking (:05), from the study of Stachodes and other 

 forms, reached the conclusion that the longitudinal canals have origi- 

 nated as inter-mesenterial chambers of a terminal polyp. The 

 absence of muscles in the walls of the longitudinal canals of Pseudo- 

 plexaura, in contrast with their presence in the mesenteries, to- 

 gether w ith the fact that the canals are sometimes traced to solenia 

 without polyps, suggests that in this form the longitudinal canals 

 have not originated in this way. Kinoshita (:10) did not succeed 

 in finding muscle fibers in the endoderm of the longitudinal canals 



