272 



THE ELASMOBRANCH FISHES 



position posterior to the apertures for the endolymphatic duets. A sagittal 

 section through the cranium of the skate as given by Howes (1883) shows that 

 each fenestra enters a well defined cavity or tympanum (ty., fig. 235b) within 

 the ear capsule, and further (fig. 235a) that the aperture is closed dorsally by 

 a membrane (nih.). Over the membrane is a semifluid layer (fl.), above which 

 is the integument. In a type in which the head is flattened (ray) this mem- 

 brane fits closely over the fenestrae, and may serve to transmit sound waves 



to the cavity below it and thus to act some- 

 thing after the fashion of the tympanic 

 membrane of the middle ear of higher 

 forms. 



The endolymphatic duct (e.d., figs. 236 

 and 237) is normally small proximally 

 and then enlarges into a more or less tor- 

 tuous endolymphatic sac (e.s.). In Squa- 

 iina, on the contrary, and to a certain 

 extent in Acanthias, the mouth of the 

 duct is enlarged and although bent at its 

 upper part it is little changed in caliber 

 throughout its course. At its base the duct 

 broadens out into the sacculus (s.). In 

 some forms this is relatively small (Alo- 

 pias: Heterodontus, fig. 236c) , but it may 

 be of large size (Sqnalus sucMii, fig. 236a, 

 s. ) . It is within this cavity as well as in the 

 utriculus that the otoliths or so-called ear 

 stones are lodged. These in Sqnalus con- 

 sist of a mass of calcareous material con- 

 tained within the endolymph. In some of the other forms they are small, and 

 in the embryo of Squatina they are apparently absent. In the adult Squatina 

 a most interesting condition is reported. Here it is said that the place of the 

 concretions of other forms is taken by sand grains which enter the wide endo- 

 lymphatic duct. 



From the inferior and posterior angle of the sacculus (figs. 236 and 237) the 

 lagena (l.) arises. This is usually a tongue-like projection as in Heptanchus, 

 but it may assume a form greatly enlarged at the end, as in Lamna. The lagena 

 is that part of the ear in the Elasmobranchs which is probably the forerunner 

 of the complex cochlea of higher forms. 



At its inferior and anterior angle the sacculus is connected with the utricu- 

 lus (h.) by the recessus utriculi. In figure 236a, the aperture from the utricu- 

 lus to the recessus utriculi is shown as a solid ellipse. The utriculus itself is 

 sometimes considered as made up of an anterior and a posterior component, 

 the anterior of which is entered by the recessus utriculi, and if seen in side 

 view may represent a T, the right and left arms of which are the horizontal 



B 



Fig. 238. Finer anatomy of an otic 

 ampulla, Acanthias. (From Retzius.) 

 A. Showing the crista acustica. B. De- 

 tail of cells. 



cr., crista acustica ; cu., cupula ter- 

 minalis; h., hair of hair cell (h.c.) ; 

 i.e., thread cell. 



