^ >UK;\ FOR MEDICAL STUDENTS CHAP. 



. ptivc apparatus, and a motor or efferent path along 

 5ent from the nerve-centre towards a muscle to make 



-In point* on the margin of the umbrella there is present a 

 flection of sensory irlls forming a sense-organ, in this case an 

 \st or primitive ear, an organ not for hearing but for performing 

 , rr ancient function o! otocysts, that of perceiving change of 

 tion in relation to the- vertical. The otocyst (Figs. 37, ot, and 39) 

 rounded sac situated on the- lower side of the rounded swollen base 

 tentacle. Tin- wall of the otocyst is very thin consisting of two 

 i . flattened cells, a covering layer of ectoderm, and a lining 

 ' ii the lining cells some have not the flattened form characteristic 

 The most conspicuous of these is a large club-shaped 

 cell which projects into the cavity and 

 vr the end of which is weighted by a large 



spherical mass of very dense calcium car- 

 oL bonate. the otolith, secreted within its cyto- 

 plasm. The otolith (Fig. 39, oT) in its 

 containing cell (o.c) rests lightly upon 

 sensory hairs which project from a patch 

 of 4-7 sensory cells (s.c) into the cavity of 

 the otocyst. 



It is clear that the action of gravity 



adifferentiated _ J 



> l upon the dense, relatively heavy, otolith 



;;:,'::""" wai cause it to bear down upon the 



sensory hairs which support it. It is 



irther that the strain upon the sensory hairs will be altered 

 if the position of the medusa be changed, e.g. if it be tilted up on edge, 

 .uparrntly in this way that the otocyst conveys to the medusa the 

 mtormation that its position has become abnormal. 



The i nelcnteric cavity is small in comparison with the bulk of the 



: .ha\ in- been lor the most part obliterated by the great development 



i. l\i-ht in the centre of the medusa a portion of the cavity 



Ltent, lorming what is usually termed the stomach since in it 



tion of the food takes place (Figs. 37 and 41, s). This 



ill the exterior by a wide four-rayed mouth at the end 



of tin- munuhrium. It also extends outwards towards the edge of the 



umbrella u tour tubes, the radial canals (Figs. 37 and 41, r.c). 



re the ribs of an ordinary umbrella by a thin 



; >K M nting coalesced portions of the endodermal 



roof and floor of the coelenteron. Around its extreme outer margin 





