jOt) 



BOUGAINVILLEA chaK 



assumption of a three-layered or Iriploblastk condition 

 (compare p. 202). 



The stem is formed of the same layers and contains a 

 cavity (citt. 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 ciitick, is developed outside 

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

 ■only stopping at the bases of the hydranths and medusa;. 

 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 medusa; 

 arc wanting. 'J'he cuticle is therefore a supporting organ 

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

 uf the bod)' {eiidoskc/c/oii, p. 16), but like the shell of a 

 r ray fish or lobster, lying altogether outside the soft parts 

 {cxoskch'/oii). 



.\s to the mode of formation of the cuticle : — we saw that 

 many organisms, such as Ha;matococcus and Amceba, are 

 able to form a c)'st or cell-wall, !>)■ 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). 15ut ^Vmceba and Hiiematococcus are 

 unicellular, and are therefore free to form this protective 

 layer at all parts of their surl'acc. The ectoderm cell.s of 



