150 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 l. 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 medusae or sessile sporosacs. In Bougainvillea 



