HYDRA. 



CHAPTER XIII. 

 TYPES OF CCELENTERATA. 



HYDRA. OBELIA. ACTINIA, ALCYONIUM. AURELIA. CYDIPPE. 



I.— HYDRA. 



Phylum 

 Class 



ccelenterata. 

 Hydrozoa. 



Fig. 45' 

 vvr 



Hydra Viridis 

 TH Two Buds 

 (Magnified). 



Hydra viridis is a small fresh- 

 water organism, which may attain 

 a length of one-half inch, but is 

 usually smaller. It is found in 

 ponds and streams attached to 

 water-weeds and is of a bright 

 green colour. 



[Hydra fusca is the brown 

 species ; with the exception of the 

 absence of green chromatophores 

 it resembles the above.] 



Hydra is axo-symmetric, hence 

 we can distinguish merely an oral 

 and an aboral end and axial and 

 peripheral parts. With the naked 



External 7^ \^ ^^^ ^^ ^^^^ ^^^^ 



Characters. ^^^ ^^^^ 'l ^l ^1^^" 

 gated cylmder fixed at 



the aboral end. At the oral end 



there is a ring of tentacles, thin 



processes which radiate in all 



directions. In the centre of this 



ring is a small raised part, the 



peristome^ upon which is situated 



the mouth. 



We may notice at once that this 



aperture, though usually termed 



the mouth, functions both as a 



mouth and an anus. 



