SENSE-ORGANS 387 



to appreciate undulatory movements in water in the shape of 

 reflex waves from contiguous surfaces or objects. 1 Their great 

 antiquity is shown by their existence in most of the Hetero- 

 straci, and in the Antiarchi and Arthrodira; although they have 

 not yet been discovered in the Osteostraci. 



The Auditory Organs. In its more typical condition each 

 auditory organ consists of a membranous sac or vestibule, 

 partially constricted into an. upper portion or utriculus and a 

 lower or sacculus (Fig. 221, A). Three semicircular canals are 

 connected with the utriculus, of which two are vertical and at 

 right angles to one another, and the third is horizontal. One end 

 of each canal is dilated into an ampulla. A slender tube, the ductus 

 endolymphaticus, leaves the sacculus, and ends in a sac-like 

 swelling, the sinus endolymphaticus, which apparently represents a 

 portion of the embryonic epidermic involution from which the 

 auditory organ is formed. A smaller sac-like outgrowth from 

 the sacculus, the lagena, corresponds to the cochlea of the 

 higher Vertebrates. The epidermic lining of this system of 

 cavities is differentiated into patches or ridges of sense-cells 

 (maculae or cristae), separated by supporting cells and innervated 

 by the terminal branches of the auditory nerve. There is a crista 

 acustica in each ampulla ; and maculae acusticae are present in 

 the utriculus, sacculus, and lagena. A fluid, the endolympli, fills 

 all the cavities, and a similar fluid or perilymph occupies the 

 spaces in the periotic capsule in which the various chambers are 

 lodged. Among the more notable deviations from this type of 

 auditory organ the Cyclostome Myxine, apparently, has but a 

 single semicircular canal with an ampulla at each end, and the 

 vestibule is a simple sac (Fig. 221, B). Petromyzon has two 

 canals, but lacks the horizontal canal. In Elasmobranchs, in- 

 cluding Chimaera (C), the ductus endolymphaticus retains its 

 primitive connexion with the exterior by means of a pore on the 

 dorsal surface of the head. In the Dipnoi (e.g. Protopterus) the 

 paired endolymphatic sinuses divide into a number of caecal 

 branches containing otoliths, which meet and interlace over the 

 fourth ventricle (Fig. 217). 2 Otoliths, either in the form of fine, 



1 Fuchs (Archivf. d. ges. Physiol. lix. 1895, p. 454) has suggested that these 

 organs may be concerned with the perception of pressure variations. It has also 

 been argued that they are concerned with equilibration and the co-ordination of 

 the movements of the fins. (See American Journ. Phys-iol, i. ]>. 128.) 



2 Burckhardt, Das Central-Nervensystem v. Protopterus, Berlin, 1892, p. 32. 



