148 INVERTEBRATE ZOOLOGY 



near the proximal end and each produces a single large ovum. 

 This is fertilized and goes through the first stages of develop- 

 ment without leaving the parent; it afterward becomes sepa- 

 rated and develops into a new individual. 



Exercise 3. Find a polyp with reproductive organs and make a 

 drawing of it. 



Special respiratory, excretory, digestive, and circulatory 

 organs are not present in 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 endoderm 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. 



No distinct muscular or nervous system is present in 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. 



