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.[452] The auditory organ must also be included as a specialised portion of this system. Both organs are innervated 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 (Fig. 219).[453] 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 subdivision of the primary pores into groups of surface-pores, complete the evolution of the system in its more advanced condition. {385}[Illustration: FIG. 219.—Vertical longitudinal section through the lateral canal of _Amia calva_. _l.n_, Lateralis nerve with its branches, _n_, _n_, to the sensory organs, _s.o_, _s.o_; _p_, _p_, _p_, external pores; _s.c_, sensory canal; _s_, _s_, scales of the lateral line. (From Wiedersheim, after Allis.)]