20 NER\'OUS SYSTEMS AND SENSE ORCIANS 



are often found, but the two kinds are not usually in the same ani- 

 mal. 



In Lizzia, the eye spots are found on the under side of the ten- 

 tacle, but in this form the tentacle is held up and its lower side 

 turned toward the litrht. 



In Poliiorchis, Little '14, the eye spot is on the outer surface 

 at the base of each tentacle. In other naked-eye medusae, similar 

 conditions are found ; the eyes may be arranged about the margin 

 as in this form, or in groups to correspond with the groups of ten- 

 tacles. 



In genus Tri()j)sis. there are eight marginal sense organs con- 

 sisting of an entodermal ocellus and an open fold of velum which 

 contains concretions. In Phopal'Dicnia the lithocy.>;ts are indo.sed. 



In the Narcomedusae there are maririnal sensory clubs con- 

 taining concretions of entodermal origin. Romanes. '98, found that 

 if the bell of a hydroid medu.'^a was removed the contractions of the 

 bell cease, but the margin which contains the nerve ring continues 

 to contract as before the injury. Any iniurv of the umbrella 

 causes no change in the rhythm so long as the rerve ring is intact. 

 The conclusion from this was that the nerve rinar is a coordinating 

 center and one needful for rhvthmical conti'actions. 



In many medusae, otocvsts or senorv clubs probably function 

 as static organs. In Anthroniedusae there are no otocysts. but 

 many have ectodermal ocelli on the bnses of the tentacles. Romanes 

 found that these had certain v'su^l functions. Medusae with them 

 were strongly attracted to light between the red and violet spec- 

 trum. 



In some forms like nouf/aivrHlia. the tentacles are grouped 

 and to correspond to each tentacle at its base is an ocellus or pig- 

 ment spot. 



In the Lentoinedusae there may be marginal sensory clubs and 

 there may be lithocysts of ec-t"dermic oriirin. In some forms such 

 as Landicca there mav be marginal sense clubs with no concretions 

 within and ectodermal ocelb at the b-^ses of the tentacles. 



In OrrJiistonia pileiis Larson, there are four hundred dark 

 brown entodermal ocelli on the circular canal : each is provided with 

 an pctodermal lens. 



ScYPHOZOANS. The marginal sense organs of this grouo arc 

 so marked as to be early recocrnized. Ehrenberg. 1837. was the first 

 to .sneak of these as organs of sense. The usual type is somewhat 

 as follows. At eight marginal notches we find two small tentacles 

 either side of a shorter hollow tentacle. This tentacle or tentaculo- 

 cvst contains otolyths and the ovfixn seems to be one of equilibrium. 

 I'non the surface of this tentnculncvst there mav he a special pig- 

 ment .spot or ocellus of rather simnle structure. In the little flap 

 above and also behind or below the short sensory tentacle there may 

 be special areas of cells which may have some sort of olfactory or 



