SENSORY CANALS OF L^M ARGUS. 83 



of the sensory canals and the habits of the fish. In the sluggish members of the Batoidei, 

 the canals are in a more or less vestigial condition ; while in the active forms they are 

 amazingly complex. For example, in Narcine hraziliensis, a small inactive torpedo found 

 often in brackish water on the east coast of Central America, in the Caribbean Sea, and 

 on the coasts of the West Indian Islands, the whole of the ventral canals are absent, and 

 the supra-orbital stops short on a level with the nasal capsule ; and while the dorsal loop 

 is present, the post-scapular branch is undeveloped. Moreover, the tubules are few in 

 number, short and simple, and the number of the sense organs is limited. 



In the torpedoes, a similar state of matters prevails. They are all, apparently, in the 

 habit of resting long on the bottom ; and with the possible exception of one or two 

 species, they seldom seem to move, save in the most sluggish fashion. As the sluggish 

 habits were gradually acquired, the extent of the sensory canal system, it may be pre- 

 sumed, has been gradually reduced. The ventral sensory canals having become useless, 

 natural selection has made no effort to preserve them, with the result that only vestiges 

 are left in the form of small vesicles — the follicles of Savi — one for each of the sense 

 organs that persist in connection with the ventral branches of the facial nerve. At the 

 same time, the dorsal sensory canals are, compared with the skate, less highly developed ; 

 the canals are simpler in their arrangement ; the post-scapulars are entirely absent ; and 

 the tubules in most species are simple, short, and few in number. 



If, from the sluggish electric rays, we turn to the active Mylobatidse, e.g., to 

 Dicerobatus, the difference in development is most striking ; the area covered by the 

 tubules and their numerous branches being far more extensive than in the common 

 skate ; the openings of the tubules being especially numerous on the ventral surface, 

 and on the dorsal surface near the margins of the pectoral fins, and at each side of the 

 middle line of the trunk. The ventral loop, which even in the skate is almost destitute 

 of tubules, has in Dicerobatus reached an extraordinary development, and throws 

 numerous branching tubules backwards towards the pelvic fin ; and there is a row of 

 tubules along the entire length of the anterior border of the pectoral fin. On the dorsal 

 surface, the branching tubules from the lateral canal are so numerous that they seem to 

 form a network at each side of the middle line — some of them even meet in the middle 

 line, and thus form trunk commissures ; and a large number of branching tubules occupy 

 the area enclosed by the dorsal pectoral loop. 



Judging from the complexity of the sensory canals, and especially from the 

 abundance of the tubules — the feelers of the canals — which they throw out in all 

 directions, it may be concluded that whatever their function, they are of the utmost 

 importance. 



It may be presumed that they take up and transmit various kinds of impressions 

 other than those which are taken cognisance of by the sense organs of the auditory 

 apparatus. Whether they enable the fish the better to obtain its food, or the better to 

 escape from its enemies, or both, remains to be made out. In comparing Dicerobatus 

 with the skate (Rata batis) and the torpedo (T. marmorata), one is especially struck with 



VOL. XXXVII. PART I. (NO. 5). 



