CHAP, v OBELIA 311 



bodies or blastostyles (bis), each bearing numerous small 

 lateral offshoots, varying greatly in form 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 enclosing 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 accordingly 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-pro- 

 cesses of Hydra there is a layer of spindle-shaped fibres 

 many times longer than broad, and provided each with 

 a nucleus. Such muscle-fibres are obviously cells greatly 

 extended in length (p. in), so that the ectoderm-cell 

 of Hydra with its continuous muscle-process 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 mesoglcea ; and so, while a hydroid polype 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 ccenosarc, 

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



