LATERAL CANAL SYSTEM OF SELACHIANS 69 



The principal canals are the supraorbital, the infraorbital, 

 and the lateral. Two shorter ones are the mandibular and 

 hyomandibular canals. The lateral canals are connected by 

 a short canal, the supratemporal commissure. Only slight 

 differences exist in the distribution of the canals in Squalus and 

 Mustelus. 



Four lines of pit-organs have been found in Mustelus and 

 five in Squalus. The dorsal series is the most extensive, in 

 Mustelus, extending from the supratemporal region to the caudal 

 extremity of the body. In Squalus this series extends only to 

 the region of the first dorsal fin. In both Squalus and Mustelus 

 two small pit organs are located immediately in front of the 

 pores of the endolymphatic ducts. Their method of develop- 

 ment gives evidence of their relationship to the canal system. 

 Two short lines of organs have been found just in front of the 

 scar left by the yolk-stalk in Mustelus pups and in Squalus 

 embryos up to 72 mm. They have not been found in the adult 

 of either species. Another series of organs common to both 

 forms extends from the lower margin of the spiracle ventral- 

 wards around the angle of the mouth and almost to the mid- 

 ventral line. The fifth series of sense organs included in this 

 group is the 'accessory line.' These organs are not strictly 

 homologous to the other organs of the group as they do not 

 have an independent development, but appear to arise from 

 the superior margin of the lateral sensory cord. Accessory 

 organs have not been found in Mustelus but it is my opinion 

 that their presence could be demonstrated, especially in early 

 embryos. 



In structure the sensory canals are epithelial tubes, one wall 

 (usually the superior) of which is specialized to form the sensory 

 epithehum. The canals lie superficially within the dermis or 

 somewhat deeper (on the head) and numerous minute tubules 

 lead from the canals to the surface, passing in a ventro-lateral 

 direction, thus permitting considerable elongation of the tubules, 

 especially in the anterior region of the body. In adult Squalus 

 and Mustelus the tubules of the lateral canal outnumber the 

 vertebrae in the ratio of approximately four to three. 



