8 Memoir Sears Foundation for Marine Research 



asphyxiation from an excess of carbon dioxide in the water, or by mechanical stimul- 

 ation. But seemingly the normal function of this reverse flow is to wash the eyes clean 

 of sand grains that may fall upon them and to clear the spiracles of fragments of seaweed 

 and other objects that may be drawn into them with the intake of water. 



Size. The members of the order range in size from a breadth of only a few inches 

 (the smallest Rajidae and Torpedo Rays) to the enormous Devil Rays that sometimes 

 grow to a breadth of 22—23 ^^^^ and to a weight of more than 3,000 pounds. The Saw- 

 fishes (Pristidae) reach a length of at least 20 feet and exceptionally even more. 



Locomotion?^ The Sawfishes (family Pristidae) propel themselves chiefly with the 

 posterior part of the trunk by lateral undulations which are effected by waves of muscular 

 contraction that progress from front to rear and thus push against the water ; this 

 action is combined with lateral strokes of the caudal fin, perhaps aided also by a sculling 

 action of the latter and by undulating movements of the pectorals.^^ Locomotion is 

 chiefly caudal also in the Guitarfishes (Rhinobatidae), though the pectorals may be of 

 some aid in their swimming." The Torpedoes swim chiefly with the posterior part of 

 the trunk, aided by the caudal fin, their discs not being flexible enough to be of much 

 service in locomotion. The Skates (Rajidae) are driven ahead by the simultaneous 

 passage of waves of undulation through the two pectorals from front to rear. Each 

 undulation originates as an elevation of the anterior margin of the pectoral, which is 

 lowered as the undulation passes rearward through the fin. The undulation increases in 

 amplitude until it reaches the midlevel of the disc, posterior to which it decreases in 

 amplitude as it continues to progress rearward. Then a new wave arises at the front of 

 the fin just before the preceding wave has entirely died away at the posterior edge. 

 In one species of Skate the time occupied by an individual undulation in its progress 

 from the front of the pectoral to the rear was 0.6-0.8 seconds. The Sting Rays (Dasya- 

 tidae and Urolophidae), as exemplified by Urolophus, use their pectorals in the same 

 way." All the batoids that do this advance by a smooth gliding motion, often right 

 along the bottom. But the Eagle, Cow-nosed, and Devil Rays (Myliobatidae, Rhino- 

 pteridae, Mobulidae) have been described repeatedly by competent eye-witnesses as 

 progressing by flapping their pectorals, more in the manner of birds. And some of them 

 have the habit of leaping into the air or of planing at the surface, a spectacular sight 

 when a school breaks the surface simultaneously, as sometimes happens (p. 485). 



The Sawfishes, Guitarfishes, and Torpedoes doubtless steer from side to side, as 

 do Sharks and most other fishes, by a progressive curvature caused by a wave of muscular 

 contraction from the anterior part of the body toward the tail, which is then swung 



15. For a general discussion of locomotion in fishes (including elasmobranchs), see Breder (Zoologica N. Y., 4, 1926: 

 159-297); for a recent analysis of the mechanics of propulsion by undulations of the trunk in various fishes, including 

 Sharks, illustrated by moving picture photographs, see J. Gray (J. exp. Biol. Lond., 10, 1933: 88-104). 



16. Breder (Zoologica N. Y., 4, 1926: 243) describes the pectorals of the Sawfishes [Pristis) as "fluttering" during swim- 

 ming and suggests that they may have some propulsive effect. 



17. In Rhitiohatos the pectorals "may be put to considerable use other than in steering, as may be seen upon grasping 

 the tail and attempting to pull the fish out of the water" (Daniel, Elasmobranch Fishes, 1934: 13). 



18. See Mcfrey (C. R. Acad. Sci. Paris, 116, 1893: 77, 2 figs.) for analysis by moving pictures of this pectoral locomotion 

 in Raja, and Daniel (Elasmobranch Fishes, 1934: 13) for Urolopkus. 



