-_ SENSE-ORGANS 387 
to appreciate undulatory movements im water in the shape of 
reflex waves from contiguous surfaces or objects 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 anampulla. 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-lke outgrowth from 
the saceulus, 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 isa crista 
acustica in each ampulla; and maculae acusticae are present in 
the utriculus, sacculus, and lagena. A fluid, the endolymph, fills 
all the cavities, and a similar fluid or perilymph occupies the 
spaces in the periotie 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- 
eluding 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).’ Otoliths, either in the form of fine, 
1 Fuchs (Archiv f. 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. Physiol. i. p. 128.) 
2 Burckhardt, Das Central-Nervensystem v. Protopterus, Berlin, 1892, p. 32. 
