DEVELOPMENT OF PLANDLA IN A PLUMULARIAN HYDEOIt). 87 



Xematophores all free and have the typic^-l plumulai-ian 

 structure ; there is a median sub-calycine nematophore, and 

 a pair of larger supra-calycine ones whi(?li scarcely project 

 beyond the level of the mouth of tlie hydrotheca (text-fig. 2). 

 The sarcothecae are bithalamic, canaliculate, and tend to be 

 narrow at the T^ase. 



Hydranth. — j^bout twenty tentacles in a single verticil. 

 Polyp constricted below the whorl of tentacles, dividing the 

 coelenteron into an upper and lower division ; the upper part 

 is lined by narrow columnar cells, the lower portion by 

 vacuolated digestive endoderm (text-fig. 3, A). 



Nematocysts in the ectoderm of the tentacles are small ; 

 they measure about 3*2 n in length and I'OQ fx in breadth. In 

 the nematophores the nematocysts are of considerable size, 

 having a length of 14*8 fx and a breadth of 3*2 /u. 



Gonosome. — The male gonangia are borne on the more 

 proximally placed pinna?. The female gonangia occur on the 

 proximal portion of the pinnate region of the main-stem ; they 

 arise just below the hydrotheca and on one side of the sub- 

 calycine nematophore. The niitritive substances are naturally 

 more abundant in the main-stem than in the lateral pinnas, 

 and from analogy with other hydroids we should expect to 

 find the female gonangia in the former rather than in the 

 latter position. This is in accordance with the view that the 

 development of the female sex is partly induced by a i*ich 

 nutritive supply. 



Male Gonangium. — The gonotheca when fully grown is 

 cylindrical, and is about 0*36 mm. in length and 0'12 mm. in 

 width (text-fig. 1, C). The perisarc is quite thin, and there 

 is no distinct operculum. At the base where it springs from 

 the pinna there is no jointed stalk, and there are no 

 nematophores arising from it. 



In the developing male gonangium the spermatic cells, 

 which will subsequently form the generative mass of the 

 single reduced gonophore, may be seen embedded in the 

 endoderm of the hydrocaulus (text-fig. 3, A, sp.c). In the 

 figure the spermatic cells are entering the blastostyle (h.G.) of 



