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TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



developmental stages of the skulls of the salmon, sturgeon, Lepidosteus, while Bigelow and 

 Welsh (1925) have figured developmental stages of many other teleosts. Larval stages and 

 fry of many species of fishes were also studied by members of the New York Zoological 

 Society's Arctur-us expedition and in Dr. William Beebe's laboratory at Bermuda. From 

 such data it is easy to trace the ontogeny of the mouth parts, suspensorium and opercular 

 region as well as of certain parts of the chondrocranium. 



In the embryonic stages of many teleosts, the eye and the brain are accelerated and 

 very large, so as to dominate the head. The mouth becomes functional about the time that 

 the yolk is used up. At first (Fig. 292) the mouth is usually very small, suited for capturing 



Sfenotomus chrysops 



Fig. 292. Stenotomus chrysops. Egg, larva and adult. After Bigelow and Welsh. 



only minute food such as diatoms or small copepods. The snout is extremely short and 

 the mouth upturned. Consequently the suspensorium is produced forward beneath the 

 very large eye. The latter depresses the palatoquadrate bars, which thus appear falsely 

 to be associated in origin with the eye (Parker, 1873, PI. I). The palatoquadrate arch of 

 embryo vertebrates is indeed called the "subocular arch" by Kesteven (1925, pp. 42-66). 

 The forward circumduction of the suspensorium brings the entire opercular fold into close 

 proximity with the eye so as to form practically the posterior border of the eye. Hence it is 

 not surprising that even after the lengthening of the space between the opercular flap and 

 the eye, the opercular series should retain a good deal of its "circumorbital" appearance. 

 This is probably the explanation of the "circumorbital" arrangement in the oldest Proto- 

 spondyli (p. 125) of the circum- and sub-orbitals, the cheek plates, preoperculars, opercular 

 and branchiostegal folds, so that traces of the embryonic arrangement persist in the adult 

 stages. The proximity of the opercular flap to the circumorbital bones suggests how readily 

 one of the latter could form an adhesion of its posterior border to the rim of the preopercular, 

 such as we find in the scorpaenoid fishes. As the individual fish became older, the space 

 between the opercular flap and the orbit would increase, perhaps to accommodate larger 

 adductor mandibulae muscles, while the contact between the nearest "suborbital" and the 

 preopercular would be retained, thus necessitating the marked lengthening of the suborbital. 

 As development proceeds, in many teleosts the snout begins to lengthen as well as the 



