MECHANORECEPTORS AND BEHAVIOR 



353 



the basic discharge, depending on the direction of the stimulus, and other 

 units that responded only on application of the stimulus. 



The Labyrinth— Because of the transparency of the cartilaginous 

 skull, and because of its size, the elasmobranch labyrinth has provided 

 significant anatomical and experimental material for the study of labyrinth 

 function in general. This organ has the same basic form in all vertebrates 

 (Figure 11) and consists of two large sacs, the sacculus and utriculus, off 

 which open the semicircular canals. Each canal, of which there are three in 

 selachians, extends from the ultriculus in an arc and bears at one end a 

 swelling (the ampulla) which houses a cupula-bearing sense organ. 



DUCTUS ENDOLYMPHATICUS 



ANTERIOR VERTICAL 

 CANAL 



CRISTAE 



UTRICULUS 



POSTERIOR VERTICAL 

 CANAL 



HORIZONTAL CANAL 



MACULA NEGLECTA 

 LAGENA 



SACCULUS 



MACULAE 



Figure 11 Simplified diagram of the elasmobranch labyrinth. 



The thin-walled utriculus houses the macula utriculi, a neuromast covered 

 by a mucilaginous cupula in which many calcium carbonate granules (the 

 otoconia) are embedded (mean size = 10 /un (Carlstrom 1963)). The sac- 

 culus, a triangular sac, also contains an otoconia-bearing sense organ, the 

 macula sacculi, and expands at one end with a protrusion, the lagena, which 

 houses another neuromast. Another sense organ, the macula neglecta, though 

 it seems to lack an otolith (Tester, Kendall, and Millisen 1972), is near the 

 junction between the posterior vertical canal and the sacculus. Dorsally the 

 sacculus opens into a narrow tube, the endolymphatic duct, which passes 

 through the chondrocranium to open to the seawater. The canal lumen 

 contains granules of calcium carbonate (sometimes sand) of the same type 

 and size as the otoconia (Carlstrom 1963). In the roof of the skull at this 



