136 PHYLUM CCELENTERATA 



Class : Hydrozoa. Order : Leptomedusce 



A CAMPANULARIAN HYDROMEDUSAN (OBELIA OR 

 CAMFANULARIA) 



These are very common marine animals which Hve in the 

 shallow water along our coast. In common with other members 

 of the group they exhibit the phenomenon of alternation of gen- 

 erations. Two generations of individuals, a sexual and an asex- 

 ual, alternate with each other. The latter is called the hydroid 

 generation ; the animal in this stage is sessile and colonial and 

 produces by budding, that is, by asexual methods, the sexual gen- 

 eration. This is called the medusoid generation ; in it the animal 

 either remains attached to the hydroid colony (Campanularia) 

 and is then called a sporosac, or separates itself (Obelia) and 

 becomes a free-swimming jellyfish, which is called a medusa; 

 in either case the medusoid produces by sexual methods embryos 

 which, after a brief free life, attach themselves to fixed objects 

 and develop into the hydroid generation. 



The Hydroid Stage. While in this stage these animals form 

 branching colonies, which are attached to seaweed, rocks, and 

 other objects. Place a small portion of a colony in a watch glass 

 in water or alcohol, and study it under the microscope. Observe 

 the differences in size between the different polyps, as well as their 

 position on the stem. Determine the method of branching. Has 

 the colony a main axial stem? If not, which is the oldest polyp? 

 Notice the ringed constrictions in various parts of the stem, espe- 

 cially near the polyp ; they give the stem strength and flexibility. 



The stem of the colony together with the branches is called the 

 hydrocaulus ; its rootlike projections by which it is attached at 

 its base are the hydrorhiza. Observe that there are two distinctly 

 different kinds of polyps instead of but one, as in Pennaria or 

 Bougainvillea : (i) the feeding polyp, or hydranth, which is the 

 more numerous and bears tentacles, and (2) the reproductive 

 polyp, or blastostyle, which is a modified hydranth and is much 

 the larger and the less numerous and bears no tentacles. Notice 

 that the perisarc, the transparent cuticular covering of the stem, 



