No. 441.] 



( X )A' ) '.!/( )RI'HA PEXDULA AG. 



5*3 



Histology of Parts. 



(a.) Hydrocaulis. — The perisarc : — The perisarc presents 

 the appearance of a thin, semi-transparent film. This film is 

 considerably thicker about the basal portion of the stem than 

 it is in the distal re-ion where it becomes so thin and delicate 



Ectoderm: — The ectoderm, although its cellular structure 

 was not especially well defined in some sections, is composed of 

 a single layer of somewhat columnar cells with large and dis- 

 tinct nuclei, and filled with numerous granules. Among the 

 cells of the ectoderm are to be found great numbers of small 

 transparent cells, the developing nematocysts. These nema- 



the tentacles. 



Entoderm : — The entoderm is composed of two distinctly 

 different kinds of cells whose disposition through the stem may 

 be best understood by calling attention here to the peculiar modi- 

 fication which Corymorpha presents as regards the enteric cav- 

 ity. In most hydroids this is represented by a hollow ccenosarc, 

 its tubular cavity being central in position and communicating 

 directly with the cavities of the polypes. In Corymorpha, how- 

 ever, this cavity is represented by a number of intercommuni- 

 cating canals running longitudinally through the entire length of 

 the stem. (Fig. i, m.) The whole central axis of the stem is 

 occupied by a column of parenchyma-like tissue composed of 

 large, loose, vacuolated cells of an irregular polygonal shape. 

 (Fig.'l, a.) ' These cells have very little contents with the excep- 

 tion of here and there a number of small nuclear corpuscles. 

 Torrey (:02) refers to these entodermal cells as -skeletal cells" ; 

 and, as he suggests, this condition of peripheral canals and a solid 

 entodermal axis, would seem to be, in view of the large diameter 

 of Corymorpha, a "direct adaptation to size." The very deli- 

 cate character of these cells might make the propriety of the 



