300 



LECTURE XII. 



The duct of the air-bladder (s) is developed frona the dorsal aspect 

 of the pyloric end of the stomach ; even in those fishes where the 

 communication of the air-duct is afterwards found at the cesophagus ; 

 the posterior compartment of the air-bladder is first developed in the 

 Cyprinoids, which accounts for the connection of the air-duct with 

 that part. In the Herring, the primitive place of its connection with 

 the alimentary canal is retained. The communication is obliterated 

 in the fishes without the air-duct ; and the whole posterior compart- 

 ment disappears with the duct in the Loach. The scales are formed 

 late in all osseous fishes ; their integuments remain smooth and 

 lubricous, as in the Dermopteri, some time after the disappearance 

 of the vitellus. 



The inferior position of the mouth is an embryonic character 

 common to all fishes, and is retained, together with the unossified 

 skeleton and the continuation of the cartilaginous vertebrte into the 

 upper lobe of the caudal fin, in all the Plagiostomes. The singular 

 productions of the rostrum in many of these fishes, like the elon- 

 gation of the jaws in osseous species, are later phenomena of deve- 

 lopment. It is interesting to find the broad, depressed, obtuse embry- 

 onic form of head common to all the known fishes of the old red- 

 sandstone. M. Agassiz thus accounts for the extreme rarity of the 

 ichthyolites of this formation presenting a profile view of the head : 

 it lies in most cases upon the vipper or the under surface. 



All the Plagiostomes have the external as well as the internal di- 

 vision of the vitellicle (Jig. 80.) ; 

 the peduncle of the external one 

 is longer, in some species consider- 

 ably so, than in osseous fishes, and 

 it is beset with villi in Carcharias 

 and Zygteiia. * The tegumentary 

 covering of the outer yolk (ib. d), 

 is denser and more opaque in Pla- 

 giostomes : the inner yolk (ib. e) 

 consists, of course, only of the 

 proper vitelline tunic, which is 

 thin and transparent : it commu- 

 nicates with the small intestine (ff) 

 i. e., with the short tract which 

 intervenes between the pylorus 

 and the valvular straight gut (h) : 

 it receives the external yolk (d' e') as this is progressively squeezed 

 into the abdomen by the contraction and interstitial absorption of its 



diess flihrt zu der Ueberzeugung, das der Fisch-Nieron stelicn geblcibunc Primor- 

 dial-nieren anderer Thieie sind." (cxxxvii. 1337, p. 314.) 

 * cxxiii. tf. iii. 



Embryo Scyllium. 



