CHAPTER XIII 



THE CCELENTERATA— HYDRA FUSCA AND 

 HYDRA VIRIDIS 



The little fresh-water polyps, known by the generic name of 

 Hydra, so common in our pools and streams, are by common 

 consent chosen as the type of the simplest form of Metazoan 

 structure. To depart from well-established custom and choose 

 another type would occasion inconvenience, so custom must 

 be adhered to in spite of the fact that Hydra is far from being 

 as simple and primitive in structure as was at one time supposed. 

 One of the simpler forms of sponges, abundant enough in 

 certain localities on our coasts, would serve better as an 

 illustration of a primitive type of multi-cellular organisation. 



Two species of Hydra are abundant in streams and pools in 

 Great Britain. The one, of a yellow or light brown colour, is 

 known as Hydra fusca. The other is called Hydra viridis 

 because of its bright green hue, due to the presence of in- 

 numerable green chlorophyll-containing corpuscles in its 

 tissues. A third nearly colourless species has been recognised 

 and named Hydra grtsea, but it differs so little and in such 

 unimportant characters from Hydra ficsca, that it need not 

 engage our attention. 



Hydra fusca being selected for descriptive purposes, the 

 general shape of the animal may be described as that of a 

 cylindrical sac forming the body, attached by one end to 

 water-weed or other object. At the opposite distal end the sac 

 opens to the exterior by a small orifice, the mouth, situated on 

 the top of a conical elevation termed the hypostome. A 

 circlet of simple, tapering, arm-like processes springs from the 

 junction of the hypostome with the body wall. These processes 

 are the tentacles. They vary in number in different specimens, 

 but there are seldom less than six and never more than nine or 

 ten. 



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