THE LATERAL SENSE ORGANS OF ELASMOBRANCHS. 475 



parent gelatinous substance, and opens on the external surface. The groups of 

 ampullae, and the proximal portions of the canals, are more or less deeply embedded 

 in connective tissue, and become visible only on first removing this. Distally, how- 

 ever, the canals lie to a large extent immediately below the skin, on removing which 

 they are revealed spreading out in all directions on both dorsal and ventral surfaces. 

 Where they pass over the muscles of the pectoral fin their course presents a series 

 of small undulations caused by the ridges and furrows of the muscle-plates. Some 

 of the canals are short, and lead in a more or less vertical direction to the external 

 surface ; others are long and horizontally spreading, in many cases diverging widely 

 as they follow their course from the ampullae to the exterior. In all cases the canals 

 terminate at the outer surface in apertures which appear as little round dots in the 

 skin, irregularly scattered or arranged in definite curved rows. The jelly contained 

 in the tubes does not appear to be naturally extruded through these openings, though 

 it can by pressure be forced out. Retzius's supposition (1847.) that the canal-openings 

 are contractile, and capable of being opened and closed at the will of the animal, does 

 not appear to have been either confirmed or refuted by any more recent physiologist ; 

 but the frequently expanded appearance of the canals towards the openings, followed 

 by a contraction at the opening itself, suggests at least a passing of the contained jelly 

 towards the exterior, and a checking of further movement as the opening is reached. 

 The groups of ampullae — or "central organs" as they have frequently been termed, 

 since they are the centres from which these canal systems arise — are constant in 

 position, and the five pairs in which they are arranged have been named by Ewart 

 after the branches of the facial nerve by which respectively they are supplied. 

 Three groups lie at no great distance from one another, embedded in the gelatinous 

 tissue of the snout. These are in some species enclosed in capsules, but not in all. 



1. The superficial ophthalmic is the most anterior group, and lies close to the 

 rostrum about mid-way between the eye and the tip of the snout (text-fig. 6). 



2. The inner buccal is situated posterior to, and to the outer side of, the preceding 

 group, and lies between it and the nasal capsule. Cases occur where this and the 

 ophthalmic lie in such close contact that, where there is no enclosing capsule, it is 

 only by means of the innervation that they can be distinguished from one another. 

 Text-fig. 3 represents a dissection of these groups in Rata clavata where this was the 

 case. The same figure also shows how the inner buccal ampullae may be distributed 

 in two sub-groups. In Raia batis, however, a single capsule encloses all the ampullae 

 innervated by the inner buccal nerve (text-fig. 7). 



3. The outer buccal group lies immediately exterior to the nasal capsule — between 

 it and the propterygium, and just in front of the antorbital cartilage (text-fig. 6). 



4. The largest and most conspicuous group is the hyoid, situated at the angle 

 between the branchial region and the inner border of the propterygium. This group 

 is in all cases enclosed in a capsular covering (text-figs. 6 to 9). 



5. The fifth or mandibular group is found on the lower jaw, slightly posterior to 



TRANS. ROY. SOC. EDIN., VOL. LI, PART II (NO. 12). 69 



