HTDROZOA. 115 



Physophoi'idw, differ, as has been shown, from 

 Galycophoridce, in possessing a float, and from 

 Medusidce in the characteristic mode by whicli 

 the polypite is connected with the rest of the 

 hydrosoma. In both the Galycoplioridce and Phy- 

 sophoridce, the nectocalyces (when present) and 

 polypites are separately attached to different parts 

 of the coenosarc. In the Medusidce, on the other 

 hand, the hydrosoma presents but one nectocalyx, 

 from the roof of which a single polypite is sus- 

 pended {fig. 4, /). The endodermal lining of the 

 polypite passes into the central cavity of the 

 swimming-organ, from which, as in other necto- 

 calyces, canals radiate, to join a circular vessel 

 surrounding the margin of the bell. From this 

 margin depend tentacles, which may be either 

 hollow processes of both layers, in immediate con- 

 nection with the canal system, or, more rarely, 

 prolongations of the gelatinous ectoderm itself. 

 Around the outer margin of the nectocalyx, be- 

 tween the endoderm of the circular vessel and its 

 ectodermal investment, are embedded the mar- 

 ginal bodies, vesicles or pigment-spots, whose 

 peculiar structure has already been described 



ififJ' 23). 



The outward form of the polypite varies greatly. 

 It may be long and highly contractile, or stoutly 

 cylindrical, or so short and broad as to be with 

 difficulty discernible on the under surface of the 

 bell {fig. 24). Very often it is curiously con- 

 stricted. In internal structure it is not known to 

 present any peculiar features. The oral margin may 

 be either simple, everted, or produced into lobes, 

 which, most frequently, are four in number, though 

 in some forms it is much divided. In Liriope 



I 2 



