CHAPTER XI 



HYDRA AND OTHER FRESH-WATER 

 HYDROZOA 



By frank smith 



Professor of Zoology and Curator of the Museum, University of Illinois 



The student of the animal life of the sea is continually in 

 contact with a great variety of organisms which have radial sym- 

 metry and are often striking in appearance, diversity, and abun- 

 dance. These were formerly included in a great group, Radiata, 

 but are now separated into two very distinct branches (phyla), 

 the Coelenterata and Echinodermata. The latter phylum, which 

 includes the well-known starfishes and sea urchins, is wholly un- 

 represented in fresh water, while the former, which includes the 

 hydroids, jellyfishes, and corals, with thousands of species in the 

 seas of to-day, has in fresh water scarcely a dozen species and 

 these are relatively insignificant in appearance. The fresh-water 

 Coelenterata are all included in the class Hydrozoa, and hydra is 

 the only one whicn is abundant, widely distributed, and well 

 known to the ordinary student of zoology. Because of its abim- 

 dance it is the type form commonly used in zoology classes as an 

 introduction to a knowledge of the phylum. 



Among the more obvious structural or morphological characters 

 of hydra is the sac-like body with the capacious chamber which 

 is at the same time body cavity and digestive cavity and of which 

 the mouth is the only opening to the exterior. The animal is 

 attached by one end and at the other shows the mouth surrounded 

 by a circle of tentacles which are evaginations of the body wall 

 and are hollow, their cavities being continuous with the digestive 

 cavity. The body wall as well as that of the tentacles is com- 

 posed of two cellular layers, the ectoderm and entoderm, sep- 

 arated by a thin, noncellular mesogloea and bounded externally 

 by a delicate cuticula. In some species there is an obvious dis- 

 tinction between an adoral part of greater diameter and more 



Si6 



