HYBRID JE. 309 



Suborder III. GYMNOCHROA. 



HYIHUNA, Ehrenberg (in part), Corall. cles roth. Meer. 67 ; Johnston, 15. 



121. 



HYDKIIKE, Huxley, Oceanic Hydrozoa, 20. 

 GYMXOTOKA (in part), .T. V. Carus, Handb. d. Zoolog. ii. ">(>!!. 



Family I. Hydridae. 

 Genus HYDRA, Linnaeus. 



])er. From uSpa, the name appropriated to the fabled Lcrmvun monster. 



GENERIC CHARACTER. Polypites locomotive, single, de- 

 stitute of polypary, cylindrical or subcylindrical, with a 

 single series of filiform tentacula round the mouth, and a 

 discoid adhesive base. Gonozooids always fixed, developed 

 in the body-walls. 



THE body of the Hydra is composed of a gelatinous and 

 highly contractile substance, and is consequently liable to 

 many changes of form. The interior is occupied by an 

 ample cavity, which extends from one extremity to the 

 other, and terminates above in a simple orifice or mouth. 

 Around this are placed a variable number of contractile 

 tentacles arranged in a single wreath. The base of the 

 body expands into a kind of disk, by which the Hydra 

 attaches itself to the stems and leaves of plants, and by 

 means of which it can also glide slowly over their surface. 

 It fulfils the contrasted functions of attachment and 

 locomotion. 



The tentacles are more or less extensile, and bear n 

 formidable armature in the shape of numerous thread-cells, 

 which arc grouped together on small prominences or 



