130 PHYLUM CCELENTERATA 



insensibility, and then swallowed into the gastrovascular 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 gastrovascular space, while by the vibrations 

 of the fiagella currents are produced which cause this fluid to cir- 

 culate 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. 



