10 



THE ELASMOBRANCH FISHES 



fish we see sufficient modification of form to insure a new mode of locomotion. 

 Here the pectorals become the elfective organs in propulsion, and the tail 

 remains at a stage of development insufficient to propel the body. The skate is 

 also an organism not only singularly adapted to its method of getting food, 

 but equally effective in crouching on the ocean floor so as to evade its enemies. 

 Last in the series we may place the sting ray, Disceus thayeri (fig. 21), 

 which even more than TJrolophus undergoes profound depression of body. In 



Fig. 19 



Fig. 21 



Fig. 20 



Fig. 19. Pristis cuspidatum. (From Garman.) 

 Fig. 20. Baia erinacea. (From Garman.) 

 Fig. 21. Disceus thayeri. (From Garman.) 



it the pectoral fins meet in front and behind so that tlie body is essentially 

 disc-shaped. Like other rays of this type its tail is provided with a serrate 

 sting, but unlike TJrolophus the tail is whip-like and has but slight indication 

 of fins or folds. 



External Form ix Its Development 



The transition in external form which we have just considered in the series of 

 sharks and rays may be similarly followed in a series of stages in the life- 

 history of an individual ray. In other words, while a ray and a shark are very 

 dissimilar in the adult, they are much alike in their earlier development. As 

 growth proceeds, the ray gradually diverges from the shark-plan so that 

 finally a fixed gulf separates the two. To illustrate this divergence we may 

 again compare Acmithias and Urolophus. 



