304 BOUGAINVILLEA chap, iv 



consistency, the branches beset with little cups, from each 

 of which, during life, a Hydra-like body is protruded. 



A very convenient genus for our purpose is Bougainvillea, 

 a hydroid polyp found in the form of little tufts a few 

 centimetres long, attached to rocks and other submarine 

 objects. Fig. 76, a, shows a colony of the natural size, b, a 

 part of it magnified : it consists of a much-branched stem of 

 a yellowish colour attached by root-like fibres to the support. 

 The branches terminate in little Hydra-like bodies called 

 hydranths (b, hyd), each with a hypostome (hyp) and circlet of 

 tentacles (/). Lateral branchlets bear bell-shaped structures 

 or meduscB {med) : these will be considered presently. 



Sections show that the hydranths have essentially the 

 structure of a Hydra, consisting of a double layer of cells 

 — ectoderm and endoderm — separated by a supporting 

 lamella or mesoglcea and enclosing a digestive cavity {e7it. 

 cav) which opens externally by a mouth placed at the 

 summit of the hypostome. 



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 1), so that the ectoderm 

 cell of Hydra with its continuous muscle-/r(?«w is here 

 represented by an ectoderm cell with an adjacent niuscle- 

 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 polyp is, like 

 Hydra, diploblastic (p. 293), it shows a tendency towards the 



