v OBELI A 309 



as Sertularians of the sea-coast, often mistaken for sea- 

 weeds : they are delicate, much-branched, semi-transparent 

 structures of a horny consistency, the branches beset with 

 little cups, from each of which, during life, a Hydra-like 

 body is protruded. 



A very convenient genus of Hydroids for our purpose is 

 Obelia, which occurs in the form of a delicate, whitish or 

 light-brown, almost fur-like growth on the wooden piles of 

 piers and wharves. It consists of branched filaments about 

 the thickness of fine sewing-cotton : of these, some are 

 closely adherent to the timber, and serve for attachment, 

 while others are given off at right angles, and present at 

 intervals short lateral branches, each terminating in a bud- 

 like enlargement. 



The structure is better seen under a low power of the 

 microscope. The organism (Fig. 78) is a colony, consisting 

 of a common stem or axis, on which are borne numerous 

 zooids (compare p. 277). The axis consists of a horizontal 

 portion, resembling a root or creeping stem, and of vertical 

 axes, which give off short lateral branches in an alternate 

 manner, bearing the zooids at their ends. At the proximal 

 ends of the vertical axes the branching often becomes more 

 complex : the offshoots of the main stem, instead of ending 

 at once in a zooid, send off branches of the third order on 

 which the zooids are borne. In many cases, also, branches 

 are found to end in simple club-like dilatations (Bd. 1,2): 

 these are immature zooids. 



The large majority of the zooids are little Hydra-like 

 bodies, the polypes or hydranths, each with a hypostome and 

 a circlet of about two dozen tentacles. Less numerous, 

 and found chiefly towards the proximal region of the colony, 

 are long cylindrical bodies or blastostyles (bls\ each bearing 

 numerous small lateral offshoots, varying greatly in form 



