xxni GENERAL CHARACTERS 251 



swimming-bells (m, in) 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, n) 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 

 that of Bougainvillea, of ectoderm, mesoglcea, and endo- 

 derm, but without a cuticle. The hydranth has a similar 

 structure to 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 prac- 

 tically medusae in which the manubrium is absent : and 

 both the bracts and grappling-lines are shown by com- 

 parison 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, '^polymorphic. 

 In addition to nutritive zooids or hydranths, it possesses 

 locomotive zooids or swimming-bells, protective zooids or 

 bracts, and tentacular zooids or grappling-lines. Morpho- 



