ZE0RH0MB1F0RMES 



473 



the middle of the lateral surface of the head. The dorsal fin may now 

 grow forward in a straight line on to the head (Pleuronectes, etc.) ; or 

 even to near the end of the snout (Rhombosolea, Gynoglossus). The line 

 marked by the fin on the head is not the true dorsal mid-line, for this 

 has become distorted on to the ocular side, as is shown by the course of 

 the lateral-line canals, and the bones of the skull (Fig. 500). The orbits 

 are far forward, and as the eye shifts, the front end of the skull (and 

 even the brain) becomes correspondingly distorted ; the relations of the 

 bones are considerably altered. For instance, the frontal of the blind 

 side is twisted round, loses its original connection, and acquires a new 

 connection with the prefrontal below, or dorsal to, the shifted eye (Fig. 

 501). The mouth, still wide and symmetrical in Psettodes, becomes 

 smaller, and more and more distorted on to the blind side, on which alone 

 the teeth may be developed. As was first clearly explained by Traquair 

 [441a], the shifting of the eyes is brought about, not by one eye leaving 



tiffi 



^■*lfa*-~ 



:.'iif 



Fig. 502. 

 Psettodes erumei, 13. and S. (After Day, Fishes of India.) 



its socket to move across to the opposite side, but by the twisting of the 

 whole anterior region of the head — an adaptation to a peculiar mode of 

 life which affects almost all the other organs as well. The branchial 

 opening becomes more restricted. The median fins become longer, ex- 

 tending forwards and backwards ; in the most specialised genera, like 

 Gynoglossus, the tail tapers to a point, the dorsal and anal fins being con- 

 fluent at the tip. Asymmetry is not pronounced in the pectoral fins ; 

 but the radials are reduced generally to mere cartilaginous vestiges, and 

 the post- temporal may lose its inner limb. The pelvic fins, bearing 

 from five to seven dermotrichia, may be distorted ; that on the ocular 

 side becoming elongated in the same line as the anal (Fig. 502). The 

 behaviour of the nostrils during torsion varies in different genera : in 

 some, Solea, the nostrils retain their position on the blind side ; in 

 Pleuronectes they migrate with the eye, but remain near the dorsal edge 

 of the lower side ; Arnoglossus has the dorsal fin passing forward below 

 them. The anus is very far forward and median ; but the urinogenital 

 papilla of the male and urinary papilla of the female are brought on 



