12 



AVERS. [Vol. VI. 



this species has budded off a sense organ.^ Tlie third open- 

 ing is used by all three canals, whose amal ends here unite into 

 the common pore opening into the remnant of the primitive 

 utriculo-saccular chamber. There is a much greater variation 

 among the vertebrate forms yet studied in the manner in which 

 the amal ends open into the auditory sac than is the case with 

 the ampullar ends, and this variation is due in large part to the 

 mechanical influences at work, shaping the auditory canal com- 

 plex during its early stages of development. Each of the canals 

 at its amal end is swollen out into a sub-oval sac, which I shall 

 designate the ama (from the Gr. ama, water-holder). This second 

 enlargement of the canals at their upper (outer) ends deserves 

 special mention from its wide distribution among vertebrates. 

 It is in no way related to sense organs, and is always lined by a 

 pavement of epithelium. It occurs in some or all of the canals 

 of the majority of Elasmobranchs ; has been figured but not 

 specially described for Teleosts ; is known to occur in the 

 human ear, and other mammalian forms in the adult stage, 

 though much more distinctly marked off during the foetal life 

 of these forms, when the double curvature of the canals, so 

 constant a feature of the fish type of canals, is also well 

 developed. 



In the Sting Ray (PI. I, Fig. i) the canal offshoots from the 

 utriculus and sacculus differ materially, as has been indicated 

 in their relations to the central parts of the canal complex. 

 Owing, in the first place, to the greater simplicity of the 

 central chambers in Dasyatis, the canals do not open into 

 the undivided median part, although there is an approach to 

 this condition in the position of the connecting tube of the 

 anterior canal complex. ' The canals of each chamber commu- 

 nicate with that chamber alone, and in this respect preserve 

 their primitive relations. 



CANALS. 



In Torpedo the semicircular canals measure 1.4 mm. in 

 diameter, the actual length of the largest canal (the anterior) 

 being 5.2 cm. Each canal lies in a relatively large channel, 

 hollowed out in the cartilaginous capsule. These canals are by 



1 The position of this organ is shown in PL I, Fig. 3, ab. and rl. ab. 



