A TUBULAR1AN HYDROMEDUSAN 149 



HYDROZOA 



A TUBULARIAN HYDROMEDUSAN. PENNARIA OR BOUGAINVILLEA 



These are marine animals, and among the commonest hydro- 

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

 which they belong, 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 

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

 sexual generation. This latter is called the medusoid genera- 

 tion; in it the animal either remains attached to the hydroid 

 colony (Pennaria) and is then called a sporosac, or separates 

 itself (Pennaria, Bougainvillea) 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 in Pennaria. 1 In this stage these animals 

 form branching colonies of polyps, which are attached to the 

 rocks 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 



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

 in the directions. 



