II 



HYDROMEDUSAE 



95 



side of each bend what looks like a tiny conical sherry-glass mounted on 

 a stalk (Fig. 36, h). In this conical receptacle there resides an individual 

 of the colony — a polyp or hydroid individual, so called because in the main 

 features of its organization it agrees with Hydra. The body wall is com- 

 posed of the same layers as in Hydra ; there is a large oral cone with a 

 wide mouth (Fig. 36, o.c), and a ring of tentacles like those of Hydra 

 except that they have a solid core of much vacuolated endoderm cells, the 

 coelenteron not extending into them. The body of the polyp is continued 

 downwards as a tube composed of the same layers and this in turn joins 

 the thread-like stolon 

 — similar in structure 

 — which meanders over 

 the surface of the stone 

 or seaweed. 



The cuticle is greatly 

 developed in Obelia 

 forming a thick horny 

 protective layer — the 

 perisarc (Fig. 36, ps). 

 This covers the whole 

 surface of the colony 

 and its branches. At 

 the base of the polyp 

 the perisarc loses its 

 intimate contact with 

 the cells of the ecto- 

 derm and expands to 

 form a wide cup — the 

 hydrotheca — which sur- 

 rounds the polyp (Fig. 

 36, h). Here and 



there in the angle between a- hydrotheca and the stem of the 

 colony there is present a vase-shaped gonotheea (Fig. 36, g). This, like 

 the ordinary hydrotheca, contains an individual of the colony but one 

 very different from the ordinary hydroid polyp. This individual — the 

 blastostyle (Fig. 36, bl) — is somewhat piston-shaped, its end being in the 

 form of a flattened disc without trace of tentacles or mouth opening. 

 Its special function is that of reproducing by budding, and numerous 

 buds may commonly be seen projecting from its surface. In their young 

 stage, as may be seen towards the lower end of the blastostyle, the 

 buds are just like those of Hydra but the older buds (Fig. 36, M), visible 



O.C. 



Fig. 36. 



Obelia. Portion of hydroid colony, x 25. bl, Blastostyle ; 

 g, gonotheea ; A, hydrotheca ; M, medusa bud ; o.c, oral cone ; 

 ps, perisarc. 



