312 OBELIA CHAP. 



tinuous with those of the hydranths : 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 for 

 protective and strengthening purposes. It is evident 

 that such a colony 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 

 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. th) 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 con- 

 traction : it suddenly withdraws itself more or less 

 completely into the iheca, 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 appear- 

 ance : 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 



