164 INVERTEBRATE ZOOLOGY 
main axial stem? If it has, which is the oldest polyp? Which 
branch is the oldest? Which is the second oldest polyp? In 
Bougainvillea, besides the polyps, the medusoids also grow out 
from the stem. 
The stem of the colony together with the branches is 
called the hydrocaulus; the root-like projections by which it 
is attached at its base are the hydrorhiza; the polyps are called 
hydranths. 
Exercise 1. Draw a diagram representing the method of branch- 
ing and showing the arrangement of the polyps on the 
stem. 
Mount a small branch of the colony on a slide in water or 
dilute glycerine, and study a number of hydranths of all sizes. 
We note the radial type of structure and the tubular body, the 
internal cavity of which opens to the outside through the small 
terminal mouth. The stem also and its branches have an internal 
cavity which is a continuation of that of the hydranth. The 
cavity of the hydranth and the stem is called the gastro-vascular 
space, and is the common digestive and circulatory cavity of 
the animal. Notice the ringed constrictions in various parts 
of the stem, especially near the polyps; they give the stem 
strength and flexibility. 
Study the two kinds of tentacles, the whorl of larger ones 
around the base of the hydranth and the shorter ones on the 
body of it. In Bougainvillea the basal tentacles alone are 
present. Count the basal tentacles. Count the short tentacles 
on a large and then on a small hydranth. The larger hydranth 
will be found to have more than the smaller one. Notice the 
small hydranth buds on the side of the stem; find one whose 
tentacles have not yet developed. Some of the hydranths will 
be seen to have large ovoid projections of very variable size on 
their sides. These are the medusoid buds, which become either 
free-swimming medusz or sessile sporosacs. In Bougainvillea 
