442 ZOOLOGY. 



The so-called " mucous canal" or lateral line of fishes and 

 Amphibians is sensory. It consists of small masses of 

 nerve-epithelium, arranged in linear series along the sides of 

 the head and body, having hair-cells continuous with nerves. 

 They are called "nerve-buttons" or " nerve-heaps." Accord- 

 ing to Schultze, their office appears to be to appreciate mass- 

 movements of the water, and more particularly vibrations, 

 which have longer periods than those appreciated by the ear 

 (Dercum). In the blind-fish of the Mammoth Cave a row 

 of sense-papilliB is situated on the front of the head, sup- 

 plied with nerve-fibres sent from the fifth pair of nerves 

 (Wyman). 



The angler {LopMus jiiscaiorius) has long been known to 

 possess hinged teeth, capable of being beut inward toward 

 the mouth, but by virtue of the elasticity of the hinge at 

 once resuming the upright position when pressure is removed 

 from them. AnaUeps and Pmcilia have also movable teeth. 

 The hake, a voracious predatory fish, and in a less degree 

 other Gadidce, are possessed of hinged teeth. 



The nature of respiration is intimately connected with the 

 production of sounds by fishes. Eecent researches by Jobert 

 on certain unusual modes of breathing in fishes are of special 

 interest. He has examined certain fishes of the Amazons, 

 i.e., species of Callichthys, Doras, ErUlirinus, Hypostomus 

 and Stidis gigas or " pirarucu" of the natives, the latter being 

 allied to the herring. In the Callichthys the intestine is trans- 

 formed into a respiratory organ. When the water dries up 

 it emigrates to other pools or streams, creeping by means 

 of its pectoral fins. This fish can live twenty-four hours 

 out of the water with impunity. 



In the gigantic pirarucu, the swimming-bladder is a long 

 sac, and the upper part does not look like that organ, being 

 spongy, areolar, reddish - brown, friable, and intimately 

 pressed to the dorsal and lateral walls of the body ; its color 

 recalls that of the lungs of a bird, and functionally it re- 

 sembles the latter. 



Among accessory breathing organs are the lamellate cavity 

 of the Afiaias, and the sac-like appendages which are in con- 

 nection with the gill-cavity, and extend under the muscles 



