A CAMPANULARIAN HYDROMEDUSAN 137 



does not end at the base of the polyp, as is the case in the tubu- 

 larian hydroid, but is continued over the polyp, inclosing it as in 

 a cup. It is thus a protective device and is called in the case of 

 the hydranth the hydrotheca, and in the case of the blastostyle 

 the gonotheca. The feeding polyp withdraws within its hydro- 

 theca for protection when alarmed. The reproductive polyp never 

 emerges from its gonotheca, which is a closed structure, but the 

 medusoids or their sexual products escape into the surrounding 

 water through an opening in the gonotheca's free end. 



Exercise 1. Draw a diagram representing the method of branching 

 of the colony and the arrangement of the polyps. 



Mount a portion of a branch with several hydranths in water or 

 dilute glycerine. Study an expanded hydranth. We note the 

 radial type of structure and the tubular body, the internal cavity 

 of which opens to the outside through the terminal mouth ; also 

 that the stem has a cavity which is continuous with that of the 

 hydranth. The internal cavity of the hydranth and of the stem 

 is called the gastrovascular space and is the common digestive 

 and circulatory cavity of the animal. The portion of the hy- 

 dranth between the base of the tentacles and the mouth is called 

 the hypostome. Count the tentacles. 



Exercise 2. Make a semidiagrammatic sketch of the expanded 



hydranth on a large scale and label all its parts. 

 Exercise 3. Make a sketch of a contracted hydranth. 



Study the finer structure of an expanded hydranth. First 

 study the structure of a tentacle. It is made up of an axis con- 

 sisting 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 re- 

 fractive nematocysts in the tentacle. These are the stinging or- 

 gans with which the animal kills its prey. Each one consists of a 

 spiral, threadlike tube, with several barbs at its base, which lies 

 coiled within the cavity of a cell called the cnidoblast. The cavity 

 is filled with a poisonous fluid ; its walls form an ovoid sac, of 

 which the tube is the very much elongated ^nd invaginated outer 



