6 ERNEST WARREN. 



projecting endodermal ridges (fig. 6, e.pr.). The ridges con- 

 sist of two kinds of endoderm cells: (1) an inner fairly 

 regular epithelium of columnar gland-cells, which stain 

 deeply (fig. 10, e. ep.), and (2) an outer, thicker layer of 

 vacuolated, i-eticular cells extending to the mesoglea (e. v. c) . 

 The cells immediately bordering the inside of the mouth 

 consist almost entirely of gland-cells (fig. 3, g. o. e.). Further 

 down, the ridges begin to flatten out, and the inner gland- 

 cells become confined to four more or less regular trefoil- 

 shaped ridges (fig. 11) somewhat recalling the mesenteric 

 filaments of an actinian. Below this, the cavity of the 

 coelenteron is widened, and the inner gland-cells are spaced 

 out (fig. 12) and do not form a distinct epithelium. 



The ectoderm at the base of the body of the hydranth, 

 where it tapers into a narrow stalk joining the coenosarc of the 

 hydrocaulus, is thrown into folds (figs. 3, 9), and here there 

 are numerous large nematocysts (8'5 fx long and 3*4 /i broad). 

 Similar nematocysts occur in the ectoderm of the hydrocaulus. 

 The calyx follows the movements of the hydranth to a 

 considerable extent. This power arises from the fact that 

 the firmer perisarc is mostly confined to longitudinal ridges, 

 which are separated by soft, flexible perisarc. The hypo- 

 stome region is covered with a thick layer of still softer 

 perisarc which permits considerable freedom of movement. 



The tentacles are capitate, and the thick perisarc on the 

 lower surface and the thin perisarc on the upper surface 

 extend to the margin of the capitulum (PI. II, figs. 16, 19, 20). 

 Running up the tentacle is an extension of the endoderm 

 from the body of the hydranth. It is frequently quite 

 typical, consisting of a single row of skeletal endoderm cells 

 (fig. 17, s. e. t.). At times, and especially in the middle region 

 of the tentacle, the cellular structure becomes lost, just as in 

 the case of the coenosarc of the distal end of the hydro- 

 caulus. The nuclei disappear, or become widely separate, 

 and the endoderm is converted into an axial, homogeneous rod 

 resembling mesoglea in its staining capacity (figs. 18, 20, 

 n. s. e. t.). 



