CHAPTER III 



PHYLUM COELENTERATA 



IT is difficult to say what idea the originator of the name 

 Coelenterata meant to convey. Most animals have hollow 

 insides (Gr. KotAos, hollow ; tWepov, inside) ; the Coelenterata how- 

 ever are distinguished from all the more highly organised groups in 

 the animal kingdom by containing inside only one set of spaces, 

 which all communicate with each other and with the exterior 

 through the mouth. 



The Coelenterate of simplest structure is undoubtedly the 

 dra common fresh-water Polyp (Hydra) (Fig. 16). If a 



mass of weed and other debris from a ditch or even 

 the edge of a river be placed in a glass vessel along with some of 

 the water in which it was grown, and allowed to settle, a number of 

 these small animals frequently termed polyps will usually be found 

 collected on the side of the vessel nearest the light. Several 

 distinct species are collected under the name Hydra. There are 

 three species recognised in Great Britain ; Hydra fusca, about a 

 third of an inch long when expanded and of a whitish yellow colour, 

 Hydra viridis, a quarter of an inch long, of a green colour, and 

 Hydra vulgaris, which is almost colourless. Similar species to the 

 first two, if indeed they are not identical, are common in Lower 

 Canada. Hydra fusca may be selected as a type. 



The shape of this animal is that of a minute cylinder. The base 

 or foot is attached to the surface of the glass by an adhesive disc, 

 whilst the other extremity carries a circle of delicate thread-like 

 appendages called tentacles. In the centre of these, near their 

 point of origin, we can with a lens detect a minute conical ele- 

 vation, the oral cone (2, Fig. 16), at the end of which is the mouth. 

 The mouth is the only opening in the body and it leads into a space 

 which occupies the whole extent of the animal, so that we might 

 s. & M. 4 , 



