90 



HYDRA. PHYLUM CCELENTERATA 



SHAPE 



The body of Hydra is a hollow cylinder, with a ring of hollow 

 outgrowths or tentacles surrounding an opening or mouth at 



one end, and the other end closed 

 by a flat basal disc or foot. The 

 mouth is raised upon an oral cone 

 or hypostome ; it leads into the 

 cavity of the cylinder, w^th 

 which the hollows of the tentacles 

 are continuous. This space is the 

 enteron. The cylinder is rather 

 wider in the middle than near 

 the ends. The wall of the body 

 is composed of two protoplasmic 

 layers (Fig. 57), the outer known 

 as the ectoderm and the inner 

 as the endoderm, with a structure- 

 less lamella or mesogloea between 

 them, consisting of a gelatinous 

 substance which they secrete. 

 Such a body as this is know^n 

 as a polyp, because of its super- 

 ficial resemblance to an octopus 

 (Latin polypus, having many feet) . 



or." 



ECTODERM 



The ectoderm consists of several 



kinds of cells (Fig. 59), of which 



the most conspicuous are those 



known as musculo- or myo- 



^ epithelial cells. These have broad 



Fig. 57 —a diagrammatic, longitud- outer ends, which meet and form 

 inai section of Hydra, magnified, the surface of the body, Standing 



— From Shipley and MacBride. . ... , . , ° 



bat., Battery of nematocysts. Only a few of OU SCVCral pillarS WhlCh rCach 



'^l%^il^erT'en^,7.'Si:L^^^ and cxpaud upou the mesogloea, 



^;;JX^':^lX^S^S^^^^ where each cell is generally con- 



■' ^*^'^"' sidered to be drawn out into one 



or more contractile processes. The processes, each containing a 



fibre, run along the cylinder and tentacles, at right angles to the 



