HYDROIDS 



479 



further, a permanent branching colony would be the result. 

 Obelia is a colony of this kind, and for its support some sort of 

 skeleton is necessary, as is the case in the corals already described. 

 Here, however, it is in the form of a horny investment, which 

 covers the common body (ccenosarc) and expands at the tip of 

 each branch into a little cup in which is lodged a hydra -like 

 polype. There is, 



B. 



A. 



TENTACLES 



however, a further 

 arrangement in the 

 form of much larger 

 cups (gonangia) 

 within which are 

 produced groups of 

 special buds, the 

 function of which 

 is to produce eggs. 

 When these buds 

 are mature they are 

 liberated in the 

 form of small jelly- 

 fish or medusce, 

 which lead an in- 

 dependent life for 

 some time and pos- 

 sess active powers 

 of movement. The 

 jelly - like consis- 

 tence in cases of 

 the kind is due to 

 the excessive de- 

 velopment of the lamella between ectoderm and endoderm, which 

 becomes thick and gelatinous, while cells from the two layers in 

 question make their way into it. The little medusa may be 

 compared to an umbrella with a very short handle (manubrium), 

 and around its margin is a fringe of tentacles, eight of which 

 have minute auditory vesicles at their bases, one to each. The 

 mouth is situated at the end of the handle, and leads into a 

 stomach, from which four tubes radiate to the edge of the umbrella, 

 where they are continuous with a circular ring -canal. It may 

 further be added that a little shelf or velum projects inwards from 



Fig. 293. Fixed and Free-swimming Stages of a Hydroid Zoophyte (Obelia}. 

 A, Natural size ; B-D, enlarged 



A, A colony of the fixed (hydroid) stage, attached to a piece of sea-weed. 

 B, End of a branch of same, c, Upper side, and D, under side of the free- 

 swimming stage (jelly-fish or medusa). 



