HYDROZOA. 



193 



The simplest type of the Hydrozoa is the genus Hydra, compris- 

 ing the Fresh-water Polypes, in which the organism consists of a 

 single tubular or cylindrical " polypite " (fig. 76), furnished at its 

 base or " proximal " extremity with a disc-like sucker or " hydro- 



Fig. 77. — a, Part of the colony of Bongainvilka mnsc7ts, one of the composite Hydrozoa, of 

 the natural size, b, Part of the same enlarged ; p, A polypite fully expanded ; m, An incompletely 

 developed medusiform bud ; ;/;', A more completely developed medusiform bud ; /, Ccenosarc with 

 its investing periderm and central canal, c, A free medusiform gonophore of the same ; «, 

 Gonocalyx ; p, Manubrium ; c, One of the radiating gastro-vascular canals ; o, Ocellus ; v, 

 Velum ; t, Tentacle. (After Allman.) 



rhiza," by which it can attach itself to foreign bodies. At the 

 opposite or " distal " end of the body is the opening of the mouth 

 (fig. 76, m), surrounded by a variable number of hollow, extensile 

 and tactile processes or " tentacles." The body-wall is composed of 

 vol. 1. N 



