GENERAL CHARACTERS 249 



Floating on the surface of the ocean in many parts of the 

 world is found a beautiful transparent organism called 

 Diphyes. It consists of a long, slender stem (Fig. 58, A, a) 

 at one end of which are attached two structures called 

 swimming-bells (;;/, m') in form something like the bowl of a 

 German pipe, while all along the stem spring at intervals 

 groups of structures (e\ one of which is shown on an enlarged 

 scale at B. 



Each group contains, first, a tubular structure (B, ?/) with 

 an expanded, trumpet-like mouth, through which food is 

 taken : this is clearly a hydranth. From the base of the 

 hydranth proceeds a single, long, branched tentacle or 

 " grappling-line " (/), abundantly provided with nematocysts. 

 Springing from the stem near the base of the hydranth is a 

 body called a medusoid (g), very like a sort of imperfect 

 medusa, and like it containing gonads. Lastly, enclosing all 

 these structures, much as the white petaloid bract of the 

 common Arum-lily encloses the flower-stalk, is a delicate 

 folded membranous plate, to which the name bract, borrowed 

 from botany, is applied. The whole organism is propelled 

 through the water by the rhythmical contraction of the 

 swimming-bells. 



Microscopic examination shows that the stem consists, like 

 those of Bougainvillea, of ectoderm, mesogloea, and endo- 

 derm, but without a cuticle. The hydranth has the same 

 structure as that of Bougainvillea, only differing in shape and 

 in the absence of tentacles round the mouth : the medusoids 

 are merely simplified medusae : the swimming-bells are practi- 

 cally medusae in which the manubrium is absent : and both 

 the bracts and grappling-lines are shown by comparison with 

 allied forms to be greatly modified, medusa-like structures. 



Diphyes is in fact a free-swimming hydroid colony which, 

 instead of being dimorphic like Bougainvillea, is polymorphic. 



