236 HYDROID POLYPES less. 



objects. Fig. 53, a, shows a colony of the natural size, n 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 medusa {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 mesogloea, and enclosing a digestive cavity {ent 

 (av) 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 (Fig. 55, end.) 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 {m./.), many times 

 longer than broad, and provided each with a nucleus. Such 

 muscle-fibres are obviously cells greatly extended in length, so 

 that the ectoderm cell of Hydra with its continuous muscle- 

 process is here represented by an ectoderm cell with an 

 adjacent muscle-<r<f//. 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. 222), it shows a tendency 

 towards the assumption of a three-layered or triploblastic 

 condition. 



The stem is formed of the same layers and contains a 

 cavity {e7it. cav') continuous with those of the hydranths, 

 and thus the structure of a hydroid polype is, so far, simply 



