l82 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



tip of the snout and innervated by the ophthalmicus superficialis 

 branch of the seventh nerve; an infraorbital line running in the same 

 way beneath the eye and supplied by the buccalis nerve; a hyomandib- 

 ular line extending along the lower jaw (and the operculum when 

 present), and innervated by the mandibularis externus; and lastly 

 the lateral line proper (sometimes double) which runs back on either 

 side to the tail and is supplied by the lateralis of the tenth nerve. 

 Frequently the systems of the two sides are connected by a supra - 

 temporal line extending across the hinder part of the skull, from one 

 side to the other. 



The lateral line organs appear in the larvae of all amphibia, but on 

 the assumption of a terrestrial life they sink beneath the skin and 

 usually degenerate, all traces of them and the lateralis nerves being lost 

 in the adult. In a few cases (Triton, Amblystoma, etc.) they are said 

 not to be entirely lost, but to reappear at the surface when the animals 

 return to the water for oviposition. Various functions have been assigned 

 to the lateral line organs. Since they contain much mucus they were 

 long called slime organs. Then they were recognized as sensory and a 

 * sixth sense ' was attributed to them. Recently it has been made very 

 probable that they are to recognize vibrations of a slow rate in the 

 water and thus, among other things, to determine currents, etc. 



Closely allied to the lateral line organs in nerve supply are the 

 ampullae of Savi and Lorenzini which occur on the head of elasmo- 

 branchs. Each consists of a long tube, opening by a pore at the surface 

 of the skin and ending with a chambered enlargement, the ampulla, 

 at the deeper end. The tube is filled with a crystal mucus and the 

 ampulla is embraced by fibres of the lateralis nerve. The organs 

 have been supposed to be connected with a pressure sense. The 

 statement is made that when they are removed the fish is unable to 

 sink; this may throw some light on their functions. 



The Auditory Organs. 



Both in character of innervation and in certain peculiarities of 

 development the sensory parts of the vertebrate ears are closely related 

 to the lateral line organs. In their most complete expression three 

 parts are recognized in the auditory organs, the outer, middle and 

 inner ears. Of these the last is the essential portion and occurs in all 

 vertebrates, the middle ear first appearing as such in the amphibia 



