SENSE ORGANS. 



185 



verberatory cavity, with a tense membrane, ty (which 

 may be compared to a membrana tympani, or, better, 

 with the membrane of the foramen ovale), a vestibular 

 sac, auv, containing otoliths (Fig. 117). In crustaceans is 

 found a similar organ, sometimes on the anterior lower 

 surface of the cephalothorax, as in crabs, and sometimes 

 on the basal joint of the antennae, as in lobsters. 



Mollusca. — In cephalopod mollusks the hearing organ 

 is in the head just below the brain, as a cavity in the 

 cartilage filled with endolymph and containing otoliths 

 (Fig. 118). In gastropods 

 it is a capsule of con- c f 



densed connective tissue 

 lined with epithelium, 

 filled with liquid and con- 

 taining otoliths, situated 

 just below the oesophag- 

 eal ganglion.* In aceph- 

 ala a similar capsule has 

 been found at the base of 

 the gills which is sup- 

 posed to have a similar 

 function. 



Below this a hearing organ has not been found. 

 Whether there be a nerve specialized for hearing is not 

 known, as we can judge only by the existence of some 

 apparatus like a capsule and otoliths. 



Thus far the most essential part of the ear is the 

 vestibular sac with its otoliths. But in spiders and 

 certain insects there is found another type of hear- 

 ing organs which may be compared not to the vestibular 

 sac, but to the hairs of the ampullae. In spiders, on the 

 feelers are found cup-shaped hollows, from the bottom 



Fig. 118. — Section through the head of 

 a squid, showing the auditory organ : 

 vs t vestibular sac and otoliths ; eg, 

 cephalic ganglion ; e, the eye. 



* Nat., iv, 518 ; Arch, des Sci., xliv, 261, 1872. 



