384 FISHES CHAP. 
lines (accessory lateral lines) or irregularly distributed. The 
“ Spalt-papillen” of Elasmobranchs are pit-organs in which the 
orifice of the pit is reduced to a sht. The more deeply-seated 
Savi's vesicles on the ventral surface 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.t’ 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 
vanals. 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 (Fig. 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 
1 See previously cited papers by Herrick and Cole ; also Ewart, Zrans. Roy. Soc. 
Edinb. xxxvi. 1892, p. 59; Collinge, Quart. Jowrn. Micr. Sci. xxxvi. 1894, p. 499 ; 
and Herrick, Jowrn. Comp. Newrology, xi. 1901, p. 177. 
2 Allis, Journ. Morph. ii. 1889, p. 463. 
