384 FISHES CHAP. 



lines (accessory lateral lines) or irregularly distributed. The 

 " SjMlt-jxqnllen " of Elasmobranchs are pit-organs in which the 

 orifice of the pit is reduced to a slit. The more deeply-seated 

 Savi's vesicles on the ventral sui'i'ace of the Torpedo, and the 

 nerve-sacs of Ganoids, are similar organs converted into closed 

 sacs and pinched off from the rest of the epidermis. Lorenzini's 

 ampullae or mucus canals, which are found in definitely located 

 groups on the lateral and upper surfaces of the head in Elasmo- 

 branchs, may perhaps be compared to pit-organs prolonged inwards 

 to form subcutaneous tubes, each of which terminates in a 

 radially- septate, chambered dilatation or ampulla, containing 

 groups of sensory cells. 



Besides the more diffusely scattered sense-organs there are 

 others which become disposed in definite lines along the sides 

 of the body and on the head, and, enclosed in grooves or closed 

 canals, constitute the highly characteristic lateral line system of 

 Cyclostomes and Fishes.^ The auditory organ must also be included 

 as a specialised portion of this system. Both organs are inner- 

 vated by the lateralis system, and both arise from a common 

 rudiment in the embryonic epidermis in the position of the 

 future auditory organ. This rudiment grows backwards along 

 the side of the body in the form of a cord of cells differentiated 

 from the epidermis, and also forwards, where it soon divides 

 into the rudiments of future supra-orbital and infra-orbital 

 canals. Sense-organs are differentiated at intervals along the 

 line of the cord ; and in the body, but not on the head, they 

 frequently exhibit a segmental disposition. Each sensory organ 

 then sinks down into a short epidermic groove, which by the 

 subsequent meeting of its lips becomes a canal detached from 

 the epidermis. The short canals then become continuous, leaving, 

 however, an externally opening primary pore between every two 

 consecutive canals, and the result is a continuous canal having 

 sense-organs imbedded in its epidermic lining and connected with 

 the exterior by pores at intervals (Eig. 219).^ The enclosure of 

 the canals in the scales of the lateral line of the trunk or in 

 special drain-pipe ossicles on the head, and the dichotomous 



^ See previously cited papers by Herrick and Cole ; also Ewart, Trans. Soy. Soc. 

 Edinh. xxxvi. 1892, p. 59 ; CoUinge, Quart. Journ. ilicr. Sci. xxxvi. 1894, p. 499 ; 

 and Herrick, Journ. Comp. Neurology, xi. 1901, p. 177. 

 2 AUis, Journ. Morph. ii. 1889, p. 463. 



