214 



CCELEXTERATA 



dermal axis formiiiL; the statoliths and the eetodermal coverin.L,' the sense cells. 

 In the .E,L,'inida' (,i and j) the elub-likc tentacles, seated on an auditory cushion, 

 Tiroject Ireeh- into the water; in the 'rrachyneniickc (2) tliey are partially trans- 

 formed into'\esieles, and in the Geryonida- they arc completely enclosed and 

 are sunk in the jcllv of the bell. 'Ehe \elar ort;ans of the Leptomcdusa? are 

 placed on the subuiiibrellar surface of the \ eluni. They may be cither simple 

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

 liths are ectodermal. I'ves and statocysts occur in dilTerent forms, a fact which 

 formerly led to a di\ ision of mcdus;t into ocellate and vesiculate groui)S. 



![mi$<i 



Fig. 177. — Statocysts (c.ir vesicles") of medus;v. 1-4, tentacular statocysts 

 Trach)'medus:v; 5, 0, \"clar of l.cptoinotUisa-; 7. marginal boi]\' of .Vcraspcdia. 1 and ^, 

 auditory clulis of .U\<^!Hi>/^s!s; 2. same of Rliof^iilcnrnni, with lieginning of ear \esicle; 4, 

 statoc}-st of i^rrv(>iil<i\ 5, *>f Arijiii>rlii: 0, auditor\' jtit of M ilioconni iiiiiur: "j . uvdvi^m^l 

 body of Aui'ilid. fk\ oetodorni; ('//, entoderm; c. mesoi:;la\i; //, auditory hairs; m, cir- 

 cular muscles cut across; ii\ n'\ up[ier and lower nerw ring; r, rini^ canal; .v, statolith. 



WHiile polyps and mcdus;x^ apparently differ so greatly from each 

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

 Hfe. The long axis of the polyji has been gretitly sJiorlciicd (tig. 178) 

 ami the cylindrical body tlevekiped into a disc; tJie mesoghra of cohimn 

 and tlisc thickened to a tliick layer of jellv; \yhile manubrial cavity, ratlial 

 and ring c;inals are renniaiits of the large gastroxasciilar space of the 

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

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



This comparison of medusa witli pol)'iJ is important in understanding 



