SENSORY CANALS OF L^MARGUS. 63 



discovered in the skate. Notwithstanding what has already been done, there does not 

 yet exist a complete and systematic account of the lateral sense organs of a single 

 Elasmobranch. As pointed out by various writers, the general anatomy of the canals 

 has been strangely neglected. When studying the sensory canals of Selachians, we have 

 still to fall back on the very meagre account of these structures given by Monro, or the 

 more elaborate, but in many respects unsatisfactory, work of Sappey. 



II. Development and General Anatomy. 



Eecent work on the origin and development of the nervous system of vertebrates has 

 necessitated, more than ever, a full account of the structure and general distribution of 

 the sensory canals in both sharks and rays, and especially of an accurate description of 

 their innervation. 



Now that the origin and distribution of the cranial nerves — more especially of the 

 trigeminal and facial — are better understood, and that the sensory canals have been 

 shown to be at the outset intimately related to certain cranial ganglia, we are in a 

 position to study their anatomy with the prospect of obtaining more valuable results 

 than was possible even a few years ago. 



Although both systems of canals have reached a remarkable development in many 

 fishes, and sensory canals or furrows with well-developed sense organs exist in nearly all 

 fishes, it has not yet been possible to determine the function of either the sensory or 

 ampullary canals. Not only is the use of the lateral sense organs still a mystery, but 

 also very little is known as to their development in Elasmobranchs. From what is 

 known, it appears to me that, in describing the. canals, special attention should be given 

 to their innervation. According to Beard (27), an epiblastic thickening is found in young 

 embryo Selachians over each visceral cleft. Towards this thickening the dorsal root of a 

 cranial nerve grows outwards from the neural crest, reaching and blending with it on a 

 level with the notochord. Where the fusion takes place, the cells proliferate and give 

 rise to the rudiment of a cranial ganglion, and, more superficially, to the rudiment of a 

 lateral (branchial) sense organ. After a time, a separation takes place ; and, eventually, 

 the deeper group of cells gives rise to a ganglion, while the superficial gives rise to the 

 sense organ. A connection between the ganglion and the sense organ is maintained by 

 a nerve (the dorsal or supra-branchial nerve), which is split off from the under surface of 

 the epiblastic thickening as development proceeds. Though, by division and growth in 

 different directions, the original simple sense organ may become complex, the dorsal 

 nerve and its branches maintain a connection between the sense organs and their related 

 ganglion. 



Taking these and other observations into consideration, together with what has 

 recently been made out as to the innervation of the sensory canals in Lsemargus and 

 Raia, it appears that, while the lateral canal of the trunk in Elasmobranchs has been 

 formed in connection with a backward growth of the lateralis division of the vagus, the 



