v OBELIA 309 



Everyone is familiar with the common Hydroids 

 known 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 deli- 

 cate, whitish or light-brown, almost fur-like growth on 

 seaweed, the wooden piles of piers, etc. 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. i, 2) : these are immature 

 zooids. 



The large majority of the zooids are little Hydra-like 

 bodies, the polypes or hydranths, each with a hypostome 

 or manubrium and a circlet of about two dozen ten- 

 tacles. Less numerous, and found chiefly towards the 

 proximal region of the colony, are long cylindrical 



