CHAP, v OBELIA 311 



according to their stage of development, and known as 

 medusa-buds (m.bd)^ these will be considered presently. 



Examination under a high power, either of an entire 

 branch or of sections, shows that the polypes have essentially 

 the structure of a Hydra, consisting of a double layer 

 of cells ectoderm (with nematocysts) and endoderm 

 separated by a supporting lamella or mesoglcea and en- 

 closing a digestive cavity (ent) which opens externally by a 

 mouth placed at the summit of the hypostome. The mouth 

 is capable of great dilatation and contraction, and accord- 

 ingly the hypostome appears now conical, now trumpet- 

 shaped. 



The tentacles, however, differ from those of Hydra in two 

 important respects. In the first place they are solid : the 

 endoderm, instead of forming a lining to a prolongation of 

 the enteron, consists of a single axial row of large cells with 

 thick cell- walls and vacuolated protoplasm. Then in the 

 position of the muscle-processes of Hydra there is a layer of 

 spindle-shaped fibres, many times longer than broad, and pro- 

 vided each with a nucleus. Such muscle-fibres are obviously 

 cells greatly extended in length (p. 1 1 i), so that the ectoderm- 

 cell of Hydra with its continuous muscle-^TW&w is here 

 represented by an ectoderm-cell with an adjacent muscle- 

 celL We thus get a partial intermediate layer of cells 

 between the ectoderm and endoderm, in addition to the 

 gelatinous mesogloea ; and so, while a hydroid polyp is, like 

 Hydra, diploblastic (p. 298), it shows a tendency towards the 

 assumption of a three-layered or triploblastic condition 

 (compare p. 202). 



The part of the stem 'and branches continuous with the 

 bases of the polypes, which is known as the ccznosarc, 

 is formed of the same layers and contains a cavity con- 

 tinuous with those of the hydranths : thus the structure of . 



