170 INVERTEBRATE ZOOLOGY 



distinctly different kinds of polyps instead of but one, as in 

 Pennaria or B o again villea : (a) the feeding polyp or hydranth, 

 which is the more numerous and bears tentacles, and (b) the 

 reproductive polyp or blastostyle, which is a modified hydranth and 

 is much the larger and the less numerous and bears no tentacles. 

 Notice that the perisarc, the transparent cuticular covering of 

 the stem, does not end at the base of the polyp, as is the case 

 in the tubularian hydroid, but is continued over the polyp, 

 enclosing 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 with- 

 draws within its hydrotheca 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 which 

 finally appears in the gonotheca's free end. 



Exercise l. Draw a diagram representing the method of branch- 

 ing 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 gastro-vascular space, and is the common 

 digestive and circulatory cavity of the animal. The proboscis- 

 like portion of the hydranth between the base of the tentacles 

 and the mouth is called the hypostome. Count the tentacles. 

 Note the absence of medusoid buds on the hydranth. 



Exercise 2. Make a semidiagrammatic sketch of the expanded 

 hydranth on a large scale and label all of its parts. 



Exercise 3. Make a sketch of a contracted hydranth. 



