312 OBELI A CHAP. 



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 for protective 

 and strengthening purposes. It is evident that such a 

 colony would, if formed only of soft ectodermal and endo- 

 dermal cells, be so weak as to be hardly able to bear its 

 own weight even in water. To remedy this a layer of trans- 

 parent, yellowish substance of horn-like consistency, called 

 the perisarc, is developed outside the ectoderm of the 

 ccenosarc, extending on to the branches and continuous 

 with a glassy, cup-like investment, or hydrotheca^ around 

 the base of each polype, and with a transparent case, or 

 gonotheca, enclosing each blastostyle. Each hydrotheca 

 (h.tti) has the form of a vase or wine-glass, and is perfectly 

 transparent and colourless. A short distance from its 

 narrow or proximal end, it is produced inwards into a sort 

 of circular shelf (sh\ perforated in the centre : upon this 

 the base of the polype rests, and through the aperture it is 

 continuous with the common stem. When irritated by a 

 touch or by the addition of alcohol or other poison the 

 polype undergoes a very marked contraction : it suddenly 

 withdraws itself more or less completely into the theca, and 

 the tentacles become greatly shortened and curved over the 

 manubrium (P. 2). At the base of each zooid or branch 

 the perisarc presents several annular constrictions, giving it 

 a ringed appearance : for the most part it is separated by an 

 interval from the ccenosarc, but processes of the latter 

 extend outwards to it at irregular intervals, and at first 

 (Bd. 2) the two layers are in close apposition. 



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 polypes, blastostyles, 



