A CAMPANULARIAN HYDROMEDUSAN 155 



HYDROZOA 



A CAMPANULARIAN HYDROMEDUSAN. OBELIA OR CAMPANULARIA 



These are very common marine animals which live in the 

 shallow water along our coast. In common with other mem- 

 bers of the group they exhibit the phenomenon of alternation 

 of generations. Two generations of individuals, a sexual and an 

 asexual, alternate with each other. The latter is called the 

 hydroid generation ; the animal in this stage is sessile and colo- 

 nial and produces by budding, i.e., by asexual methods, the 

 sexual generation. This latter 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 jelly-fish, which is called 

 a medusa; in either case the medusoid produces by sexual 

 methods embryos which 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, especially near the polyp ; they give the stem 

 strength and flexibility. 



The stem of the colony together with the branches is called 

 the hydrocaulus ; its root-like projections by which it is attached 

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



