NOTES ON MEDIAN AND PAIRED FINS OP FISH. o45 



are fifteen trunk segments between the pectoral and the 

 pelvic nerve-plexus in Esox lucius^ three in Cyprinus tinea, 

 and none at all in Gadus. To account for this by v, Jhering's 

 theory, we must suppose that a new trunk, presumably also 

 new viscera, have developed behind the pelvic fins, while the 

 old trunk and viscera have disappeared in front ! Moreover, 

 in Lepidoleprus and Uranoscopus the 3rd spinal nerve shares 

 in both the pectoral and the pelvic plexus. 



Still more difficult to explain by excalation and inter- 

 calation is the case of the Elasmobranchs. There are twenty- 

 three segments between the pectoral and the pelvic plexus 

 in Zygtena, only three in Pristis, and none at all in many 

 Rajidfe; yet, of course, the other parts remain unalfected. 



The evidence of embryology is also thoroughly opposed to 

 such a theory. Comparing various forms, such as Rana 

 with Necturus, Lacerta with a snake, etc.; we find large, 

 sometimes vast, differences in the number of segments ; we 

 might, therefore, expect to discover in the embryo zones 

 where segments are either being formed or absorbed. Not a 

 trace occurs of such zones of growth or absorption. 



The nerve-plexus of the pectoral fin of Spinax occupies ten 

 segments, that of Torpedo twenty-seven, that of Trygon fifty- 

 seven : no sio-n whatever of zones of excalation or intercalation 

 has been found in their development. It is unnecessary to 

 multiply instances (Fiirbringer 11, Brans 3). 



But if it is difficult to account for the varying position of 

 the paired limbs on the theory of excalation and intercala- 

 tion, the task becomes impossible if we attempt thus to 

 explain the varying position of both the paired fins and the 

 unpaired fins; for we find that the various fins alter in 

 position and extent independently of each other. 

 No scheme of excalation and intercalation, however ingeni- 

 ously devised, can ever account for the position of the first 

 dorsal fin opposite the pectoral in Lamna, between the 

 pectoral and the pelvic fins in Alopecias, opposite the pelvic 

 in Scyllium, and well behind it in Raja. 



Returning, now, to the other explanation of the change of 



