132 PHYLUM CCELENTERATA 



Exercise 1. Draw a diagram representing the method of branching 

 and showing the arrangement of the polyps on the stem. 



Mount a small branch of the colony on a slide in water and 

 study a number of hydranths of all sizes. 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 contin- 

 uation of that of the hydranth. The cavity of the hydranth and 

 the stem is called the gastrovascular 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 variable size on their sides. These are the medusoid buds, 

 which become either free-swimming medusae or sessile sporosacs. 

 In Bougainvillea the medusoid buds do not appear on the hy- 

 dranths but on the stem ; they become free-swimming medusae. 



Exercise 2. Make a semidiagrammatic sketch of a large hydranth 

 and a portion of the stem on a large scale ; label the different parts. 



Exercise 3. Make a sketch on a large scale of the smallest hydranth 

 or hydranth bud you can find. 



Mount a hydranth and a part of the stem on a slide in dilute 

 glycerine, and study their finer structure. First study the struc- 

 ture of a tentacle. It is not hollow as is the tentacle of Hydra, 

 but is made up of an axis consisting of a single row of large 

 entoderm cells around which is a layer of small ectoderm cells. 

 Between these two cell layers is the delicate, noncellular supporting 

 layer. Find the highly refractive stinging organs, or nematocysts, 

 at the end of the tentacle. These are the organs with which the 



