CHAPTER 11 



PHYLUM H. : COELENTERATA 



This phylum includes all those multicellular animals in 

 which the cells are massed closely together, side by side, to 

 form a two-layered body-wall, enclosing a single body-cavity 

 with a single terminal aperture. The body is radially 

 symmetrical, and definite organs and tissues are developed. 

 Because of the stinging hairs which nearly all these forms 

 can eject, they are popularly known as stinging animals, or 

 Lasso-throwers, or on account of their sedentary life and 

 curious forms they are sometimes called zoophytes, the plant- 

 like animals. 



Class I. : HYDROZOA 

 Type : The Hydra or Fresh-water Polyp. 



This form, like most of those studied in the last chapter, is 

 commonly found in pond water, but it is easily to be seen 

 with the naked eye, for the body in some species, when 

 stretched out to its full length, is frequently as much as 

 ^ of an inch long. 



General The thread-like tubular body is usually attached 



Form. by its lower closed end to some object in the 

 water, a water-weed or piece of stick ; the free end narrows 

 slightly, but is open, forming a definite mouth. At a short 

 distance below the mouth, there springs from the body a 

 circle of 6 to 8 arms or tentacles, which are hollow like the 

 body, their cavities being merely continuations of the body- 

 cavity. The whole body is very sensitive to toucli, changing 

 its shape very rapidly, so that from a long thread-like form 

 with extended tentacles (Fig. 9, A) it quickly contracts into a 



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