A CAMPANULARIAN HYDROMEDUSAN 169 
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, #.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 
