Fig. 68.—Isocercal tail of Hake, Merluccius productus (Ayres).
Fig. 69.—Homocercal tail of a Flounder, Paralichthys californicus.
The part of the body of the fish which lies behind the vent is known as the urosome. The urostyle is the name given to a modified bony structure, originally the end of the notochord, turned upward in most fishes. The term opisthure is suggested by Ryder for the exserted tip of the vertebral column, which in some larvæ (Lepisosteus) and in some adult fishes (Fistularia, Chimæra) projects beyond the caudal fin. The urosome, or posterior part of the body, must be regarded as a product of evolution and specialization, its function being largely that of locomotion. In the theoretically 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.
Fig. 70.—Gephyrocercal tail of Mola mola (Linnæus). (After Ryder.)
Homologies of the Pectoral Limb.—Dr. Gill has made an elaborate attempt to work out the homologies of the bones of the pectoral limb.[4] From his thesis we take the following:
"The following are assumed as premises that will be granted by all zootomists: