250 



HYDROID POLYPES 



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- 





^Jri'Tb ' s * -^ - . * v -\ - '- 

 ^ Vo^X'.V-:''. . 



' ". >V *?*..' ^'-V^^ . .- - 



^$'%$'h$$l'{i $ '> i'.'f ^ ' ; ,i) Hi &ife\!^.' : ' :";,'' 



^llil ; &yW 



--,;/., ; 



FIG. 59. A, Porpita pacified (nat. size), from beneath, showing disc- 

 like stem surrounded by tentacles (/), a single functional hydranth (/$j), 

 and numerous mouthless hydranths (/y 1 ). 



B, vertical section of P. mediterranea, showing the relative positions 

 of the functional (hy) and mouthless (Ay 1 ) hydranths, the tentacles (/), 

 and the chambered shell. (A after Duperrey ; B from Huxley after 

 Kolliker.) 



logical and physiological differentiation are thus carried 

 much further than in such a form as Bougainvillea. 



Porpita is another free-swimming Hydroid, presenting at 



