212 



CCELENTERATA 



All other important organs arise from the ectoderm. Gonads arise 

 in many species (fig. 176 from the ectoderm of the manubrium; in others 



A 



Fir,. 175. Rhopalonema velatum. c, ring canal; e, exumbrella; , gonads; 7z, oto- 

 cysts; m, stomach; n, nerve ring; o, mouth; s, subumbrella; t r , /', tentacles of first and 

 second order; v, velum. 



from the same layer covering the subumbrellar surface of the radial 

 canals (fig. 175), forming in either case conspicuous, often orange or red, 

 thickenings. Longitudinal ectodermal muscles move the tentacles in a 



