XXII DIPH\^ES AND PORPITA 249 



" grappling-line " (t), 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 dract. 

 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 diflFering in shape 

 and in the absence of tentacles round the mouth : the grap- 

 pling lines are formed on the polype-type : the medusoids are 

 merely simplified medusae : the swimming-bells are practic- 

 ally medusae in which the manubrium is absent : and the 

 bracts 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. 

 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- 

 logical and physiological differentiation are thus carried 

 much further than in such a form as Bougainvillea. 



Porpita is another free-swimming Hydroid, presenting at 

 first sight no resemblance whatever to Diphyes. It has much 

 the appearance of a flattened medusa (Fig. 59), consisting 

 of a circular disc, slightly convex above and concave below, 

 bearing round its edge a number of close-set tentacles, and 

 on its under side a central tubular organ {hy) with a ter- 



