BIMEKIA iJKilDA SP. N. 



9 



is formed in them, and this is the beginning- of the ectoderm- 

 chamber (PI. I, fig. 2, e. c). The whole hydranth is still 

 uniformly covered with perisarc, but that forming the lateral 

 walls begins to thicken and will ultimately form the cup or 

 calyx. The ectoderm of the polyp is in close contact with 

 the calyx-wall at this stage. 



The tentacles elongate and the cuticle or perisarc over 

 the capitula becomes evanescent. On the calyx some trace of 

 longitudinal ridges continuous with the tentacles can now be 

 observed (text-fig. 2, a). The young hydranth grows, and 

 a second series of tentacles arises alternating with the first 

 series, and additional longitudinal ridges on the calyx-wall 

 originate between those which are already present (b). When 

 the tentacles of the second set have grown considerably a 

 third series is produced (c). 



In a cross-section thi'ough the calyx, smaller ridges alter- 

 nating with larger ones may be noticed ; the former belong- 

 to the last-formed tentacles (PI. I, fig. 7, r. y. t.). The older, 

 larger tentacles are sometimes held more upright than the 

 younger, shorter ones which may tend towards the horizontal 

 position. Thus the tentacles of the hydranth may be alter- 

 nately raised and depressed, a common feature in many 

 hydroids (text-fig. 1, y.h.). 



The subtentacular region of the body gradually contracts 

 from the calyx, leaving a space which increases as the polyp 

 grows older (figs. 7 and 9, e. cy.). A mouth is formed between 

 the ectoderm-chamber and the coelenteron, and the chamber 

 communicates with the exterior by a central aperture. 



The perisai-c now thickens until the adult condition is 

 attained. 



As the polyp grows older the lumen in the distal ccenosarc 

 of the hydrocaulus becomes constricted (fig. 8), and sometimes 

 nothing but mesoglea may remain. 



Gonosome. — The reproductive bodies are in the form of 

 fixed gonophores surrounded by a firm layer of perisarc. 

 Only the male gonophore has been discovered. The gono- 

 phores are oval or rounded in outline, and they are carried 



