TELEOSTEI, C:> 



not appear to be any such indications as in Elasmobranchii of the 

 paired fins arising as parts of a continuous lateral fin. 



Most osseous fishes pass through more or less considerable post-em- 

 bryonic changes, the most remarkable of which are those undergone by the 

 Pleuronectidse '. These fishes, which in the adult state have the eyes 

 unsym metrically placed on one side of the head, leave the egg like normal 

 Teleostei. In the majority of cases as they become older the eye on the 

 side, which in the adult is without an eye, travels a little forward and 

 then gradually rotates over the dorsal .side of the head, till filially it conies 

 to lie on the same side as the other eye. During this process the rotating 

 eye always remains at the surface and continues functional ; and on the 

 two eyes coming to the same side of the head the side of the body without 

 an organ of vision loses its pigment cells, and becomes colourless. 



The dorsal fin, after the rotation of the eye, grows forward beyond 

 the level of the eyes. In the genus Plagusia (Steenstrup, Agassiz, No. 56) the 

 dorsal fin grows forward before the rotation of the eye (the right eye in 

 this form), and causes some modifications in the process. The eye in 

 travelling round gradually sinks into the tissues of the head, at the base 

 of the fin above the frontal bone ; and in this process the original large 

 opening of the orbit becomes much reduced. Soon a freidi opening on 

 the opposite and left side of the dorsal fin. is formed ; so that the orbit has 

 two external openings one on the left and one on the right side. The 

 original one on the right soon atrophies, and the eye passes through the 

 tissues at the base of the dorsal fin completely to the left side. 



The rotating eye may be either the right or the left according to the 

 species. 



The most remarkable feature in which the young of a large number of 

 Teleostei differ from the adults is the possession of provisional spines, very 

 often formed as osseous spinous projections the spaces between which 

 become filled up in the adult. These processes are probably, as suggested 

 by Giinther, secondary developments acquired, like the Zouea spines of 

 larval Crustaceans, for purposes of defence. 



The yolk-sack varies greatly in size in the different types of 



rr, , - J 



leleostei. 



According as it is enclosed within the body-wall, or forms a distinct 

 ventral appendage, it is spoken of by Von Baer as an internal or external 

 yolk-sack. By Von Baer the yolk-sack is stated to remain in communica- 

 tion with the intestine immediately behind the liver, while Lerebouliet 

 states that there is a vitelline pedicle opening between the stomach and 

 the liver which persists till the absorption of the yolk-sack. My own 

 observations do not fully confirm either of these statements for the Salmon 

 and Trout. So far as I have been able to make out, all communication 

 between the yolk-sack and the alimentary tract is completely obliterated 

 very early. In the Trout the communication, between the two is shut off 

 before hatching, and in the just-hatched Salmon I can find no trace of any 

 vitelliue pedicle. r jhe absorption of the yolk would stem therefore to be 

 effected entirely by blood-vessels. 



1 Vide Agassiz (No. 56) and Steenstrup, Malm. 

 B. E. II. 5 



