MUSEUM OF COMPARATIVE ZOOLOGY. 65 



system in its relations to the general structure and to the habits of 

 the species, and, through the latter, to the surroundings and to their 

 influence upon its evolution and variation. Direct opportunity for 

 much of this is not within reach ; but from the material at hand 

 it may be possible to make approximatious that at least will be 

 tolerable. 



Possession of the organ is quite general ; no exception has yet been 

 discovered either in the Selachia or in the Holocephala. A stage of 

 development of the system that is comparatively simple exists on those 

 forms usuallv called the lower, and on the course from them to the 

 highest the amount of complexity is found to correspond well with the 

 rank as indicated by the brain or other parts of the organization. 

 Between Heptabranchias and Alopias of the Galei, and between Pristis 

 and Dicerobatus of the Batoidei, each step is marked by variation in 

 contour and in the extent and complication of the system of the canals. 

 In the Holocephala a blow is apparently given to the idea that the 

 groove is the lowest, the primitive form of canal, by the fact that Callo- 

 rhynchus possesses tubes and not furrows like those of Chimsera. This 

 may be no more than an indication that the former is the most differ- 

 entiated type, the higher in rank. Bony Fishes, also, possess tubes. 

 The fact remains that it is among the lower forms of the Sharks and in 

 Chimaera that the grooves obtain. Furrows are unknown in the canals 

 of the Batoidei ; and it is in this order that the greatest degree of devel- 

 opment is attained by the system. Dichotomization of the tubules ap- 

 pears in the higher, and becomes excessive in the highest, forms of either 

 order. Types known to be sluggish in their habits are less abundantly 

 supplied with tubules, and the system is not so complex as on the more 

 active. It needs but a contrast of the Raiidae and the Myliobatidae 

 to make this obvious. Forms which have changed their habits and 

 become more addicted to resting on the bottom give evidence of the fact 

 in the gi'adual deterioration and disruption of certain canals on the 

 under portions of the body. That the canals are rather less subject 

 to variation, that is, that they respond less quickly to its causes, than 

 certain other organs, is intimated by the results of a comparison of 

 the species of a single genus. Close genetic relationship is asserted 

 by the canals of such species as Dasybatus walga and D. nudus, or by 

 Uraptera agassizii and Raia Icevis. Young specimens, or embryos, often 

 serve better as guides to descent and affinity than old ones, since canals 

 are present on the former which in some instances can hardly be found 

 on the older ones. In the embryo the canal system takes on its generic 



VOL. XVII. — NO. 2. 5 



