192 ME. F, J. COLE ON THE STRUCTUEE AND MOEPHOLOGY OF 



each other, but in later stages they differ greatly, the pit organ retaining its embryo- 

 logical form and individuality, while the canal organ first increases greatly in size, 

 and then, by the independent growth of other similar organs immediately adjoining it, 

 gives rise to a large sensory patch, or nerve ridge (Merkel), in which the separate 

 organs lose to a greater or less extent their individuality." Again, on p. 629, lie 

 says : — " A line of pit organs represents ajoparently the possibility of a canal ; with 

 the disappearanee of the organs the possibility of the canal, even in rudiment, would 

 certainly disappear." 



A striking fact in the comparative anatomy of the sensory canals that has hitherto 

 been overlooked is the relative abundance of the mucus secreted. Starting with the 

 Cyclostome fishes, we find most elaborate mucous organs and a very imperfectly developed 

 lateral line system. In Myxine the mucous sacs occupy the position of the lateral or 

 body canal in the higher fishes, and I think it must be agreed with Eisig and Dohrn 

 that these sacs represent very archaic structures which must have existed before the 

 lateral canal system itself. In the Elasmobranch fishes we get a condition ia which 

 mucous and sensory elements are combined, but in which the sensory elements as it were 

 have developed at the expense of the mucous elements. In other words there is a com- 

 bination of sensory and secretory cells, but the latter now exercise a subordinate (though 

 prominent) instead of a predominant function. Above the cartilaginous fishes the m.ucous 

 elements become more and more reduced, until the minimum is reached when tliey are 

 reduced to a few secretory cells at the base of each sense organ. Further, in the recent 

 TeleostSj the sense organs themselves are unmistakably reduced in number — in some cases 

 very considerably. The lateral sense organs were, as is well known, for a long time 

 regarded as secretory organs. On the discovery, however, of sense organs in the canals, 

 opinion veered romid to the other extreme, and completely ignored the undoubted 

 occurrence of glandular structures in the lateral canals and ampulhe. This is the further 

 surprising since the existence of such structures is to be expected. The other sense 

 organs of the head — the nose, the eye, and the ear — have all accessory glands connected 

 with them, and the physiological necessities of the lateral sense organs postulated the 

 existence of glandular cells liere also. Now the glandular organs ai)pcar to have played 

 at first a predominant, and then an accessory part in the history of the lateral 

 line organs, and it consequently follows that if they were at first predominant 

 they were also pre-existent and independent structures. There is therefore, in my 

 opinion, some justification for the view that the sensory and secretory portions of the 

 lateral line system were at first independent of each other, and that the incorporation of 

 the latter into the former necessarily followed on the adoption of the canal in preference 

 to the superficial type of sense organ. The rapidly evolving lateral line system, as it sank 

 below the surface, seized on the glandular organs and subordinated them to its own 

 use. But what is this use ? I think it is sufficiently obvious. From tlie structure and 

 consistency of the lateral canal mucus, it follows that it has considerably departed from 

 its original function of lubricating the surface of the body, and can no longer perform 

 that function. In the lateral canals it forms a delicate jelly, the "shivering" of which 

 agitates the sensory hairs and thus couvc) s a message from witiiout. It thus corresponds 

 precisely to the endolymph of the ear, is physiologically unnecessary in a superficial 



