SENSORY CANALS OF THE COMMON SKATE. 89 



dorsal surface it curves backwards, and eventually terminates by communicating with a 

 lono- offshoot from the lateral canal. 



The two lateral canals, or canals of the lateralis divisions of the vagus nerves, begin 

 on a level with the spiracle (sp., fig. 6), and extend backwards to the tip of the tail, 

 giving off branches which unite and form the temporal commissure, and also two long 

 (scapular) branches, the anterior of which communicates with the dorsal extension of the 

 hyomandibular to form a wide dorsal loop. 



I. The Supra-Orbital Canal. — This canal not only communicates by its proximal end 

 with the infra-orbital canal as in Lsemargus, but also with the anterior end of the lateral 

 canal, as in Chlamydoselachus, Acanthias, and certain other sharks. Beginning on a 

 level with the anterior margin of the spiracle, as shown in figure 6, it first arches inwards 

 and then runs forwards parallel to the middle line as far as the nasal capsule ; it then 

 inclines inwards as it proceeds to the tip of the snout (S.O. 2 , fig. 6), which it pierces, and 

 thus reaches the ventral surface, to run backwards and slightly outwards, as shown in 

 figure 7. When some distance from the nasal aperture, it bends forwards and outwards 

 to form a close loop (S.O. 4 , fig. 7), and finally runs outwards and backwards in front 

 of the nasal capsule, to terminate by opening into the infra-orbital canal (S.O. 5 , fig. 7) 

 Notwithstanding the great difference which obtains between the skate and Lsemargus, 

 the supra-orbital canal in the skate only really differs from that of Lasmargus in being in 

 direct connection with the canal of the lateral line, and in presenting a well-marked loop 

 before it unites with the infra-orbital. 



The supra-orbital canal, as above described, includes the cranial, rostral, and subrostral 

 canals of Garman (5). The first (cranial) part of the supra-orbital lies immediately under 

 the skin, and in some cases occupies a shallow but well-marked groove in the cranial 

 cartilage. It has an internal diameter of 0'5 mm., while the entire canal is oval in form, 

 measuring about 4 mm. by 2 mm. in thickness. Seven short, slender tubules spring from 

 the first part of the canal (t.-t. 7 , fig. 6), and run inwards to open by minute pores about 

 1'2 cm. from the middle line. From the next part of the canal, nine slender tubules 

 (t.-t. 16 , fig. 6), only one of which is over 3 mm. in length, extend directly outwards, and 

 from the part of the canal that lies over the nasal capsule there are seven tubules (t.-t. 23 , 

 fig. 6) which run outwards and forwards, the three anterior ones being over 2 cm. in 

 length. Passing to the first section of the canal are sixteen nerves, the majority of which 

 at once penetrate the canal, but the anterior ones break up and supply the sense organs 

 corresponding to the anterior tubules. There are probably altogether twenty-three ter- 

 minal branches. 



The part of the supra-orbital canal which runs along the rostrum (rostral canal of 

 Garman; S.O^-S.O. 2 , fig. 6) differs from the part just described, and from the cranial 

 canals in Lsemargus, in giving off no tubules — in having no direct communication with 

 the exterior. It lies about 2 mm. below the skin in the subcutaneous tissue, and has 

 throughout the greater part of its length a diameter of 27 mm. It has thus a diameter 

 fully five times that of the cranial portion, while the wall is only about 1 mm. in thick- 



