3o6 BOUGAINVILLEA chap. 



assumption of a three-layered or triplobtastic condition 

 (compare p. 202). 



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 polyp 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. 76, a, would, if formed 

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

 to be hardly able to bear 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 medusae. 

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

 is left as a semi-transparent branched structure resembling 

 the living colony in form except that hydranths and medusae 

 are wanting. The cuticle is therefore a supporting organ 

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

 'of the body {endoskeleton, p. 16), but like the shell of a 

 •crayfish or lobster, lying altogether outside the soft parts 

 (exoskeleton). 



As to the mode of formation of the cuticle : — we saw that 

 many organisms, such as Haematococcus and Amoeba, are 

 able to form a cyst or cell-wall, by secreting or separating 

 from the surface of the protoplasm a succession of layers 

 either of cellulose or of a transparent horn-like substance 

 (pp. 232 and 244). But Amoeba and Haematococcus are 

 unicellular, and are therefore free to form this protective 

 layer at all parts of their surface. The ectoderm cells of 



