214 



CGELENTERATA 



dermal axis forming the statoliths and the ectodermal covering the sense cells. 

 In the yEginidae (i and 3) the club-like tentacles, seated on an auditory cushion, 

 project freely into the water; in the Trachynemidae (2) they are partially trans- 

 formed into vesicles, and in the Geryonidse they are completely enclosed and 

 are sunk in the jelly of the bell. The velar organs of the Leptomedusae are 

 placed on the subumbrellar surface of the velum. They may be either simple 

 pits, or the mouths of the pits may close. In these both sense cells and stato- 

 liths are ectodermal. Eyes and statocysts occur in different forms, a fact which 

 formerly led to a division of medusae into ocellate and vesiculate groups. 



1. 



ea.- 



m 



$06. 



m. - 



FIG. 177. Statocysts (ear vesicles) of medusae. 1-4, tentacular statocysts of 

 Trachymedusse; 5, 6, velar of Leptomedusae; 7, marginal body of Acraspedia. i and 3, 

 auditory clubs of Aeginopsis; 2, same of Rhopalonema, with beginning of ear vesicle; 4, 

 statocyst of Gerycmia; 5, of Aequoria; 6, auditory pit of Mitrocoma annir; 7, marginal 

 body of Aurelia. ek, ectoderm; en, entoderm; g, mesogloea; h, auditory hairs; m, cir- 

 cular muscles cut across; l , n~, upper and lower nerve ring; r, ring canal; s, statolith. 



While polyps and medusae apparently differ so greatly from each 

 other, the medusae are only highly modified polyps adapted to a swimming 

 life. The long axis of the polyp has been greatly shortened (fig. 178) 

 and the cylindrical body developed into a disc; the mesoglcea of column 

 and disc thickened to a thick layer of jelly; while manubrial cavity, radial 

 and ring canals are remnants of the large gastrovascular space of the 

 polyp, obliterated in the other regions by the pressure of the mesoglcea. 

 To the parts thus formed only the velum and sense organs are added. 



This comparison of medusa with polyp is important in understanding 



