162 INVERTEBRATE ZOOLOGY 
near the proximal end, and each produces a single large ovum. 
The green hydra is a hermaphroditic animal, having both testes 
and ovary when sexually active, while the brown hydra is 
diccious, being either male or female. 
Exercise 3. Find a polyp with reproductive organs and make a 
drawing of it. 
Special respiratory, excretory, digestive, and circulatory 
organs are absent in the hydra. Respiration and excretion are 
carried on through the surface of the body-wall. Digestion, 
circulation, and absorption go on within the gastro-vascular space. 
The animal’s prey is caught in the water with the tentacles, 
which sting it into insensibility, and then swallowed into the 
gastro-vascular space. The entoderm cells, which line this 
cavity, extend their very plastic inner ends toward or about 
it, and some of them secrete a digestive fluid. The object is 
then digested and mingles with the water present in the gastro- 
vascular space, while by the vibrations of the flagella currents 
are produced which cause this fluid to circulate throughout all 
the parts of it. 
Distinct muscular and nervous systems are absent in the hydra. 
Delicate muscle fibers are present, however, in the form of long 
parallel projections of the inner ends of ectoderm cells. Nervous 
elements are also present in the form of isolated ganglionic cells 
situated in the ectoderm, which send delicate projections to the 
muscle fibers and to the nematocysts. Both muscle fibers and 
ganglion cells are present throughout the animal’s body but 
are most numerous in the tentacles. There are no organs of 
special sense. 
