( 87 ) 



VI. — On the Lateral Sense Organs of Elasmobranchs. II. The Sensory Canals of the 

 Common Skate (Eaia batis). By J. C. Ewart, M.D., Eegius Professor of Natural 

 History, and J. C. Mitchell, B.Sc, University of Edinburgh. (Plate III.) 



(Read 21st December, 1891.) 



Introductory. 



In the paper on the sensory canals of Lsemargus, communicated to the Royal Society 

 in July last, it was pointed out that the arrangement of the sensory canals differed con- 

 siderably in the Batoidei from that in Selachoidei, and it was mentioned that the sensory 

 canals of the skate would be next described. 



The skate has been selected chiefly because the sensory canals are more typical than 

 in the torpedoes and the whip and sting rays. In the torpedoes some of the canals are 

 in a vestigial condition ; while in the rays proper they have in most species undergone 

 great specialisation. But another reason is, that an account of the development of the 

 lateral sense organs in the skate is likely ere long to be published ; and, further, the skate 

 will be more easily obtained and kept under observation than the rays, when physiologists 

 eventually direct their attention to the lateral sense organs of Elasmobranchs. 



In describing the sensory canals of the skate, an acquaintance with the papers on 

 the sensory canals (1) and the cranial nerves (2) of Lsernargus will be taken for granted. 



The General Anatomy and Innervation of the Sensory Canals. 



The lateral sense organs in the skate consist of (1) sensory canals; (2) ampullary 

 canals ; and (3) sensory follicles or pit organs. As in Lsernargus, the sensory canals may 

 be said to consist of four main canals — viz., (l) a supra-orbital; (2) an infra-orbital; (3) 

 a hyomandibular ; and (4) a lateral canal. The ampullary canals radiate from five 

 centres, their inner dilated ends giving rise to five groups of ampullae. . One, the most 

 posterior (hyoid) group of ampullae, lies at the outer end of the hyomandibular cartilage ; 

 a second (the superficial ophthalmic) group lies at the side of the rostrum ; a third (the 

 inner buccal) group lies in front of the nasal capsule ; a fourth (the outer buccal) group 

 lies in front of the antorbital cartilage ; while the fifth (the mandibular) group lies near 

 the outer end of the mandible. The sensory follicles (" spalt-papillen " of Fritsch) consist 

 of three rows of shallow pits, one of which lies internal to the canal of the lateral line, a 

 second row lies under the orbit, and a third row near the auditory pores. 



In the Lsernargus paper it is pointed out that Stenonis discovered the openings of 

 the mucous (ampullary) canals of the skate in 1664, and that both the ampullary and 

 sensory canals were discovered some years later (1678) in the torpedo by Lorenzini, who, 



VOL. XXXVII. PART I. (NO. 6). P 



