LESS, xxiii STRUCTURE OF A HYDRA^TH 239 



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 

 medusae (med) : these will be considered presently. 



Sections show that the hydranths have just 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 (ent. cav.) which 

 opens externally by a mouth placed at the summit of the 

 hypostome. 



The stem is formed of the same layers and contains a 

 cavity (ent. cav') continuous with those of the 'hydranths, 

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

 that of a Hydra in which the process of budding has 

 gone on to an indefinite extent and without separation of 

 the buds. 



There is however an additional layer added in the stem 

 for protective and strengthening purposes. It is evident 

 that a colony of the size shown in Fig. 54, A would, if formed 

 only of soft ectodermal and endodermal cells, be so weak as 

 to be hardly able to b^ar its own weight even in water. To 

 remedy this a layer of transparent, yellowish substance of 

 horny consistency, called the cuticle, is developed outside 

 the ectoderm of the stem, extending on to the branches and 

 only stopping at the bases of the hydranths and medusse. 

 It is this layer which, when the organism dies and decays, 

 is left as a semi-transparent branched structure resembling 

 the living colony in all but the absence of hydranths and 

 medusae. The cuticle is therefore a supporting organ or 

 skeleton, not like our own bones formed in the interior of 



