Morphology of the Fins 



The part of the body of the fish which Hes behind the vent 

 is known as the urosome. The urostyle is the name given 

 to a modified bony structure, originaUy the end of the noto- 

 chord, turned upward in most fishes. The term opistJiure 

 is suggested by Ryder for the 

 exserted tip of the vertebral 

 column, which in some larvas 

 {Lepisosteus) and in some adult 

 fishes (Fistularia, CJt-iiuccra) pro- 

 jects beyond the caudal fin. The 

 urosome, or posterior part of the 

 body, must be regarded as a prod- 

 uct of evolution and ' specializa- 

 tion, its function being largely 

 that of locomotion. In the theo- 

 retically primitive fish there is no 

 urosome, the alimentary canal, as 

 in the worm, beginning at one end 

 of the body and terminating at 

 the other. 



Homologies of the Pectoral Limb. 

 — Dr. Gill has made an elaborate 

 attempt to work out the homol- 

 ogies of the bones of the pectoral 

 limb.* From his thesis we take 

 the following: 



"The following are assumed as premises that will be granted 

 by all zootomists: 



" I. Homologies of parts are best determinable, cateris pari- 

 bus, in the most nearly related forms. 



"2. Identification should proceed from a central or deter- 

 minate point outwards. 



"The applications of these principles are embodied in the 

 following conclusions : 



" I. The forms that are best comparable and that are most 

 nearly related to each other are the Dipnoi, an order of fishes 

 at present represented by Lepidosiren, Protopterus, and Cera- 



FiG. 70. — Gephyrocercal tail of Mola 

 mola (LinnEPUs). (After Ryder.) 



* Catalogue of the Families of Fishes, 1872. 



