84 PROFESSOR J. C. EWART ON THE 



the difference in the condition of the sensory canals of the ventral surface. In the 

 torpedo the ventral loop is entirely absent, and there are only a few follicles under the 

 snout, representing the anterior canals. In the skate, the ventral loop and the various 

 ventral portions of the cranial canals are well developed, but the tubules are extremely 

 few in number — the long loop only possessing nine in all. In Dicerobatus, as pointed 

 out above, the ventral loop and its tubules have reached a state of great elaboration. 

 These differences might very well be accounted for by the differences in the mode of life 

 of the three forms. The torpedo moves about little, and trusts both for its defence and 

 its food to its batteries ; and the sensory canals are, as it were, only sufficiently developed 

 to enable it to appreciate ordinary disturbances in its immediate vicinity. 



The skate moves about more than the torpedo, but less than Dicerobatus ; but in its 

 movements it usually keeps close to the bottom, and apparently it has comparatively few 

 enemies. It is far less often captured by other fish than one would naturally suppose ; 

 and it seems to have little difficulty in obtaining its food. Hence, though the ventral 

 loop is present, it possesses few tubules ; and, though the dorsal canals are well developed, 

 the tubules are simple and less numerous than in Dicerobatus. 



As to the habits of Dicerobatus, little is known ; but it undoubtedly differs from both 

 the torpedo and the skate. Instead of resting almost constantly on, or merely skimming 

 along the bottom, it is in the habit of taking long flights through the water. This prob- 

 ably accounts for the great development of the tubules of the ventral loop. Why the 

 dorsal tubules are so enormously complex is more difficult to account for ; but their 

 development may have some relation to the unprotected condition of the tail, which is 

 not only devoid of a spine, but comparatively short and powerless. 



Taking into consideration the marked difference between the sensory canals of the 

 sharks and those of the skates and rays, it will be well, before dealing further with the 

 sharks, to give an account of the sensory canals in one of the Batoidei — this I hope to do 

 in a subsequent communication. 



[For Explanation of Plates see page 102.) 



[Bibliography. 



