80 FORMATION OF THE TAIL. 



by neural and haemal processes, forms the permanent tail fin, 

 which though internally unsymmetrical, assumes an externally 

 symmetrical form. The upturned end of the notochord which 

 was originally continued into the primitive dorsal lobe becomes 

 enshcathed in a bone without a division into separate vertebrae. 

 This bone forms the urostyle (u). The haemal processes belong- 

 ing to it are represented by two cartilaginous masses, which 

 subsequently ossify, forming the hypural bones, and supporting 

 the primary fin rays of the tail (fig. 36 C). The ultimate 

 changes of the notochord and urostyle vary very considerably in 

 the different types of Teleostei. Teleostei may fairly be 

 described as passing through an Elasmobranch stage or a stage 

 like that of most pre-jurassic Ganoids or the Sturgeon as far as 

 concerns their caudal fin. 



The anterior paired fins arise before the posterior ; and there 

 do 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-embry- 

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

 Pleuronectidae 1 . These fishes, which in the adult state have the eyes 

 unsymmetrically 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 finally it comes 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 fresh 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. 



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



