X INTRODUCTION. 



The tentacles or prehensile organs are ranged round the 

 oral extremity, or variously distributed over the surface of 

 the body. They are filiform appendages, more or less ex- 

 tensile, and always bear a formidable armature of thread- 

 cells, which are often aggregated in prominent groups, 

 so as to roughen the surface, or to give it a beaded 

 appearance. 



The tentacle of the Hydroid is a tubular extension of the 

 wall of the body, and communicates at the base with its 

 cavity. In some cases, as in Hydra, it seems to be a simple 

 tube, open throughout, in which the fluids circulate freely ; 

 but generally the cavity is more or less obliterated, and the 

 tentacle presents the appearance of being septate, and par- 

 tially filled up by a cellular axis. 



In two of the suborders under which the Hydroida are 

 ranged, the tentacles are disposed in a single wreath or 

 circle, which surrounds the base of the proboscis, and are 

 simply filiform ; but amongst the Athecata they exhibit 

 considerable variety both of form and arrangement. 

 They are sometimes scattered over the body of the poly- 

 pite, in some cases there is a slight tendency to a spiral 

 arrangement, in others they are placed in two remote 

 circles, and in one species, at least, their number is reduced 

 to two. Amongst the Tubulariida a large number of very 

 short arms immediately surround the oral extremity, and 

 a wreath of long slender tentacles encircles the base of 

 the body. In many species these organs are arranged in 

 two approximate series, one immediately behind the other, 

 so closely set as to appear like a single circlet. 



In this suborder the tentacles are either slightly clavate 

 or capitate, or simply filiform. In some genera the capi- 

 tate and filiform kinds are both present. 



