SENSORY ORGANS AND RECEPTION 



341 



Invertebrates. A few invertebrates are reported to be sensitive to sounds 

 and water-borne vibrations. Underwater sounds evoke withdrawal 

 (startle) reflexes in sabellid worms, e.g. Branchiomma. Some crustaceans 

 produce sounds, e.g. Palinurus, Synalpheus, Uca (p. 405), and may have 

 limited auditory ability. It is known that Mysis and Palaemonetes can hear 

 underwater sounds. Bethe observed that they are more responsive to deep 

 tones and that sensitivity is reduced after extirpation of the statocysts. 

 It has been suggested that the crustacean statocyst, although primarily a 

 statoreceptor, functions also as a rudimentary phonoreceptor, like the 

 sacculus of fishes. 



Hearing in Fishes and Function of the Lateral Line System. Fishes detect 

 sound waves and low-frequency (infrasonic) vibrations by means of the 

 inner ear, lateral line and cutaneous receptors. The vestibule of the inner 



(a-) C b) 



Fig. 8.26. (a) Labyrinth of Exocoetus. (b) Section through the 

 Sacculus of the Trout 



cp, posterior semicircular canal; /, statolith of lagena; lag, lagena; n, nerve; s, sacculus; 

 se, sensory epithelium; //, utriculus. ((a), after Plate (121); (b), after von Frisch (51).) 



ear contains a membranous sac, the sacculus, from which arise two small 

 evaginations, the utricle and lagena (Fig. 8.26). The latter is homologous 

 with the cochlea of higher vertebrates. The epithelial lining of these 

 chambers bears small patches of sense cells termed cristae or maculae. 

 Calcareous statocysts are present in the three chambers, suspended by 

 membranes so that they are able to vibrate freely. During vibration they 

 stimulate the sensory epithelium. The maculus of the lagena is supplied by 

 a separate branch of the eighth cranial nerve. 



A connexion between the air-bladder and ear exists in some teleosts. In 

 the simplest cases a diverticulum from the air-bladder extends to a mem- 

 branous fenestra in the periotic capsule so that vibrations, picked up by the 

 bladder, can be transmitted to the perilymph (tarpon Megalops, soldier- 

 fish Holocentrus, etc.) In the Clupeidae the diverticulum enters the periotic 

 capsule and becomes closely associated with the labyrinth. The most com- 

 plex arrangement is found in the essentially freshwater Ostariophysi which 

 possess a series of movable bones, the Weberian ossicles, extending from 

 the air-bladder to the perilymph (44). 



