HYDROIDS AND JELLY-FISH 43 



Less numerous than the flower-like polyps, but dotted here and 

 there about the colony, are long cylindrical bodies (blastostyles), 1 

 each enclosed in a transparent urn or vase-shaped case (gonotheca), 2 

 and bearing numerous roundish upshoots, which vary considerably in 

 shape according to their stage of development, and called " medusa 

 buds." Now, the tentacle-crowned polyps are the feeding or nutri- 

 tive polyps (nutritive zooids), whose duty it is to obtain and 

 digest food for the general growth and upkeep of the colony ; 

 while the less numerous generative polyps (blastostyles) form 

 and carry the medusa buds (generative zooids). These buds, at 

 first, are mere hollow offshoots of the generative polyp (blasto- 

 style), but as they develop they assume the appearance of tiny 

 saucers attached by the middle of their convex surface, the edge 

 of each saucer bearing a fringe of some sixteen short tentacles ; 

 while a curious blunt process, like a sort of bell-clapper (called the 

 manubrium), and terminating in an opening, or mouth, projects 

 from the centre of the concave surface of each saucer. Ultimately 

 the little saucers become detached from the generative polyp or 

 blastostyle, and, making their escape through an aperture at the 

 top of the transparent gonophore which has served them as a kind 

 of nursery, they swim away as tiny medusas or jelly-fish. 



These tiny medusae or jelly-fish of Obelia are very beautiful 

 and interesting little creatures. They swim about by rhythmic 

 movements of their saucer or umbrella-shaped bodies, and in their 

 progress through the water the little umbrella may become turned 

 inside out, exposing the manubrium and the four-sided mouth to 

 view. However, this is not a serious accident, and the little jelly- 

 fish soon rights itself, for it has no brittle ribs to crack like those 

 of a real umbrella. Although such a small, transparent, gelatin- 

 ous creature, the little* medusa is quite a complex animal. The 

 margin of the umbrella is furnished with a varying number of 

 tentacles, and in many cases with a series of small sacs enclosing 

 one or more refractive spherules, probably rudimentary sense 

 organs ; while at the base of the tentacles there is often a collec- 

 tion of pigment cells in which a crystalline body is embedded, 

 the whole forming a coloured spot or ocellus, which may function as 



1 Greek blastos, a bud ; stylos, a column : columniform zooids destined to give 

 origin to generative buds. 



Greek gonos, offspring : a receptacle for the generative buds. 



