SENSOR? CANALS OF L^EMAEGUS. 81 



compound nerve, and made up of four sets of dorsal fibres — the dorsal branches of vagus 

 II. i III., IV., and V. In my scheme, which represents the arrangement in Raia and 

 Lamna, the lateralis nerve is represented as springing by a special set of fibres on a 

 higher level and partly in front of the glossopharyngeal, and it supplies not only the 

 lateral line (the lateral canal of the trunk), but also the temporal commissure and a 

 portion of the great lateral canal in front of the commissure. The lateralis either receives 

 from or gives slender branches to the four or five divisions of the vagus as it proceeds 

 backwards ; but whether this implies that all the divisions of the vagus have contributed 

 to the formation of the lateralis, I am not yet in a position to say. Strangely enough, 

 the lateralis has a special and distinct ganglion. 



Vagus I., II., and III., in the case of the skate, are all separate nerves, and each has a 

 ganglion, visceral, pre- and post- branchial branches. Vagus IV., which has also the 

 usual branches, is intimately connected with the intestinal division, but the ganglia are 

 partly or completely separate. In the skate there are thus in connection with the vagus 

 six ganglia. The value of these, and especially the origin of the ganglion of the lateralis, 

 whether it arises independently or from vagus I., or from all the branchial divisions of the 

 vagus, are points well worth further investigation. 



It is worthy of note that all the groups of ampullae — the superficial ophthalmic 

 (S.O.A.), inner (I.B.A.) and outer (O.B.A.) buccal, hyoid (H.A.) and mandibular (M.A., 

 fig. 2) — are supplied by dorsal branches of the facial, and that the lateralis, in addition 

 to supplying the entire length of the lateral canal, innervates (e.g., in Mustelus and 

 Raia) a row of pit organs (p.od., fig. 2). 



Concluding Observations. 



Although from a morphological point of view there seems no doubt that the sensory 

 canals subserve some important function, it has not yet been discovered by actual 

 experiment what this function is. 



Garman (29) thinks that it has been pretty well established that the function of the 

 sensory canals is " that of very delicate tactile organs receiving and carrying the slighter 

 vibrations of the water, noting changes of density, currents, &c." 



It is now most desirable that experiments should be instituted, under favourable 

 conditions, to test the various suggestions made as to the function of both the sensory 

 and ampullary canals. In view of further experiments being undertaken, I cannot do 

 better, in concluding this paper, than point out some of the more striking modifications 

 of the sensory canals found in Elasmobranchs.* 



In the first place, in some of the sharks, as already noted, a furrow takes the place of the 

 greater extent of the lateral canal. In Chlamydoselachus, e.g. , the lateral canal, with the 

 exception of a small part behind the temporal commissure, is represented by a groove, 

 guarded by overlapping scales. In Heptanchus maculatus, the canal opens into a groove 



* For outline figures showing the arrangement of the canals, see Garman (29). 



