236 



CCELENTERATA. 



All other important organs arise from the ectoderm. Gonads 

 arise in many sj^ecies (fig. 170) from tlie ectoderm of the manu- 

 brium; in others from the same layer covering the subumbrellar 

 surface of the radial canals (fig. 178), forming in either case con- 

 spicuous, often orange or red, thickenings. Longitudinal ectoder- 

 mal muscles move the tentacles in a snaky fashion, whence the 

 name medusa. Circular striped muscles run on the subumbrellar 

 side of bell and velum, causing the characteristic motion. By this 

 contraction the bell becomes more arched and narrowed, while the 



D 



Fig. 180.— Otocysts of Medusfe.— .4, Cunina ; B, Rhopolnnema : C Carmarinn (Trachy- 

 medusse) ; /A Octorcliis (Leptomedusan). o, epithelium ; ;i, auditory ceUs ; W, 

 origin of hairs ; hh, auditory hairs ; hp, auditory cushion ; .. otolitha • n audi- 

 tory nerve ; ni\ nerve ring. 



velum (which hangs down when at rest — fig. 178, A) contracts 

 like a diaphragm across the mouth of the l)ell (lig. 178, B). Since 

 water is thus forced out through the opening the medusa is forced 

 forward by the reaction. 



The circular muscles of umbrella and velum are separated by 

 the nerve ring, with which arc connected the sensory organs- 

 eyes of the simplest type: red pigment spots with or without a 

 lens (fig. ,S1); and open or closed auditory vesicles (otocysts). 

 Tactile hairs are abundant on the tentacles. 



