A TUBULARIAN HYDROMEDUSAN 131 



Class : Hydrozoa. Order : Anthomedusm 

 A TUBULARIAN HYDROMEDUSAN (PENNARIA) 



Pennaria is a marine animal and one of the commonest hydro- 

 medusans along our coast. As is characteristic of the group to 

 which it belongs, it exhibits 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 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 and is then called 

 a sporosac, or separates itself 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 

 hfe, attach themselves to fixed objects and develop into the 

 hydroid generation. 



The Hydroid Stage in Pennaria.^ In this stage these animals 

 form branching colonies of polyps, which are attached to the rocks 

 and seaweed in shallow water. The colonies are several inches 

 in length, and are found in thick clusters which often cover the 

 rocks over small spaces ; their color is a delicate pink. 



Place a small portion of a colony in a watch glass of water 

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

 main stem of the colony and its branches, also the position of 

 the polyps on the branches. Note carefully the differences in 

 size between the different polyps. Which is the largest polyp? 

 Study the method of branching. Has the colony a main axial 

 stem? If it has, the oldest polyp will be the one at the tip of 

 the stem and the second oldest will be the one at the end of the 

 lowest branch. 



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

 hydrocaulus ; the rootlike projections by which it is attached at 

 its base are the hydrorhiza ; the polyps are called hydranths. 



1 Bougainvillea or any other tubularian can also be used, with slight changes in 

 the directions.- 



