420 



INTRODUCTION TO CRANIATA 



[CH. 



narrow connection with the exterior throughout life 'through a long 

 tube called the ductus endolymphaticus (2, Fig. 209). In the 

 higher forms this tube is still recognisable but no longer opens to 

 the exterior. Each pit contains a watery fluid termed endolymph 

 and becomes constricted in the middle into an upper portion, the 

 utriculus, and a lower portion, the sacculus. In all Craniata 

 except the Cyclostomata the former gives rise to three flat out- 

 growths placed in planes at right angles to one another (Fig. 209). 

 These outgrowths become converted into half-rings by the meeting 

 of their walls in the middle of each, and in this way three semi- 

 circular canals are formed, called respectively anterior, posterior 

 and horizontal. The primary function of the whole organ, like that 

 of the otocysts of Medusae, Crustacea and Mollusca, is to enable 



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FIG. 210. Section of ampulla of the internal ear. 



1 Sense-cell bearing a long hair. 2. Sense-hair. 3. Nerve termi- 

 nation branching round base of sense-cell (dendrites of a deeply placed 

 neuron). 4. Interstitial cell. 5. Gelatinous cup in which the 

 sense -hairs are embedded. 



the animal to perceive its position. Where each semicircular 

 canal arises from the utriculus it is swollen, and the swelling 

 is termed an ampulla. The wall of each ampulla projects 

 inwards, and the projection contains cells with exceedingly long 

 sense-hairs which project into the cavity of the ampulla (Fig. 210). 

 The free ends of these hairs are embedded in a gelatinous cup 

 which sways about with the motion of the endolymph and thus 

 stimulates the sense-cells of the ampulla pressing on their sense- 

 hairs as on levers. It can be thus seen that the whole organ is 

 admirably adapted to record change of position in any direction, 

 since any change of position can be completely analysed into 

 movements in three planes. The lower part of the organ or 



