98 ZOOLOGY FOR MEDICAL STUDENTS chap. 



a sensory or perceptive apparatus, and a motor or efferent path along 

 which impulses are sent from the nerve-centre towards a muscle to make 

 it contract. 



At eight points on the margin of the umbrella there is present a 

 special collection of sensory cells forming a sense-organ, in this case an 

 otocyst or primitive ear, an organ not for hearing but for performing 

 the far more ancient function of otocysts, that of perceiving change of 

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

 is a rounded sac situated on the lower side of the rounded swollen base 

 of a tentacle. The wall of the otocyst is very thin consisting of two 

 layers of greatly flattened cells, a covering layer of ectoderm, and a lining 

 layer. Of the lining cells some have not the flattened form characteristic 

 of the majority. The most conspicuous of these is a large club-shaped 



cell which projects into the cavity and 



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, ol) 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 



'^'^^ 39- the otocyst. 



OtocystofOfitfZia, highly magnified, t.. ■ i ._i ._ . i ,■ r 



seenfromabove. <.c, Undigerentiated It IS clear that the action of gravity 



cells lining cavity of otocyst ; o.c, cell upon the dense, relatively heavy, otolith 



which secretes otolith in its interior; .,, 



ol, otolith ; s.c, sensory ceUs. W"! cause it to bear down upon the 



sensory hairs which support it. It is 

 clear further 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. 

 It is apparently in this way that the otocyst conveys to the medusa the 

 information that its position has become abnormal. 



The coelenteric cavity is small in comparison with the bulk of the 

 medusa, having been for the most part obliterated by the great development 

 of mesogloea. Right in the centre of the medusa a portion of the cavjty 

 remains patent, forming what is usually termed the stomach since in it 

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

 communicates with the exterior by a wide four-rayed mouth at the end 

 of the manubrium. It also extends outwards towards the edge of the 

 umbrella as four tubes, the radial canals (Figs. 37 and 41, r.c). 

 These are connected as are the ribs of an ordinary umbrella by a thin 

 membrane — in this case representing coalesced portions of the en,dodermal 

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



