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Show 1895.] SENSOEY CANAL SYSTEM OF FISHES. 287 sides of the walls of each cavity there arise a series of sensory filaments (PL X I X . figs. 8 and 9) into which a fine branch of a nerve passes. In Schismatorhynchus heterorhynchus, Leydig (8. p. 2) describes the walls as showing folds as figured (Taf. i. fig. 5). In Labeo these folds were scarcely discernible, probably owing to the fact that the material had been for some time in alcohol. These interesting organs seem to m e to be a series of specialized cluster-pores which have become isolated from the sensory canal system. There are a number of other " sense-papilla?," as Leydig terms them, common to certain Cyprinidee, which have also probably originated as cluster-pores. IV. ESOCIDJE. Esox LUCIUS. General Description. The sensory canal system of Esox has been previously investigated or referred to by Leydig (7), M'Donnell (9), Ramsay Wright (12), Allis (1), and others. The system is a simple one and shows none of the complications previously met with in the Siluroids or Cyprinoids. The lateral canal, like all the canals in Esox, is a wide tube passing from the posterior end of the body to the anterior, and joins the main canal of the head by passing through the supra-clavicle and a somewhat Y-shaped canal-bone; the lateral arm connects the lateral canal of the trunk with the main canal of the head, which passes through the lateral border of the pterotic, and on the sphenotic divides into supra- and suborbital branches. The former passes through the frontal to a point slightly anterior to the lateral ethmoid, and then into a canal-bone lying on the lateral border of the anterior portion of the frontal. It terminates anterior to the nasal capsule some distance from the end of the snout. The suborbital passes over the sphenotic in a small ovoid canal-bone and is conducted around the orbit in a series of canal-bones. In front of the orbit it makes an upward turn and terminates at a pore lateral to the nasal capsule. Passing through the preoperculum is a fairly large canal, which has no connection with the main canal or with that in the mandible. This last mentioned commences on the ventral surface of the mandible a little anterior to the articular portion, and passes to almost the end of the ramus. It is not connected with the branch of the opposite side. In addition to the above canals and branches there are in Esox a number of open grooves upon the surface of the head posterior to the occipital region and in the nasal region. Course of the Canals and Branches. 1. T H E L A T E E A L C A N A L . - T h e presence of a canal on the lateral trunk of the body seems to have escaped the notice of previous |