170 INVERTEBRATE ZOOLOGY 
distinctly different kinds of polyps instead of but one, as in 
Pennaria or Bougainvillea: (a) the feeding polyp or hydranth, 
which is the more numerous and bears tentacles, and (6) 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 inacup. 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 1. 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. 
