RELATIONS OF TEMPERATURE TO VERTEBRAE AMONG FISHES. 



BY 



David Starr Jordan, 



PresiJeiif of Lcland Stanford, Jr., University. 



It has been known for many years that in certain groups of fishes the 

 northern or cokl-water representatives have a larger number of verte- 

 braj than those members which are found in tropical regions. To this 

 generabzation, first fornmlated by Dr. Gill in 18G3, we may add cer- 

 tain others which have been more or less fully appreciated by ichthyol- 

 ogists, but which for the most part hav^e never received formal state- 

 ment. In groups containing fresh water and marine members, the 

 fresh-water forms have in general more vertebr.ne than those found in 

 the sea. The fishes inhabiting the depths of the sea have more verte- 

 brjie than their relatives living near the shore. In free-swimming pela- 

 gic fishes the number of vertebra^, is also greater than in the related 

 shore fishes of the same regions. The lishes of the earlier geological 

 periods have for the most part numerous vertebrae, and those fishes 

 with the low numbers (24 to 26) found in the specialized spinj^-rayed 

 fishes appear only in comparatively recent times. In the same connec- 

 tion we may also bear in mind the fact that those types of fishes (soft- 

 rayed and anacanthine) which are properly characterized by increased 

 numbers of vertebrai r>redominate in the fresh waters, the deep seas, 

 i and in Arctic and Antarctic regions. On the other hand the spiuy- 

 ^, rayed* fishes are in the tropics largely in the majority. 



*Fortlio purpose of tlie present discnssion, wemay regarrl the ordinary fishes, ex- 



chisive of sharks, ganoids, eels, and other primitive or aberrant types as forming 



i! three categories : (1) The soft-rayed or Physostomous fishes, with no true spines in 



\\ the fius, with an open duct to the air-bladder, the ventral fins abdominal (the pelvis 



:1 being attached only by the flesh and remote from the shoulder-girdle), cycloid 



i scales, etc. (2) The spiny-rayed or Acanthopterygiau fislies, having usually spiues 



in the dorsal and other fins, no duct to the air-bladder, the skeleton firm, the ven- 



trals attached by the pelvis to the shoulder-girdle, the shoulder-girdle joined to the 



skull, and the scales usually ctenoid or otherwise peculiar. The vertebra; among 



; spiny-rayed fislies are larger, and therefore generally fewer in number, and their 



appendages (shoulder-girdle, gill arches, ribs, interspinal bones, etc.) are more 



specialized. The spiny-rayed fishes are usually regarded as the most specialized or 



"highest" in the scale of development. The question of whether, on the whole, 



they are "higher" or "lower" as compared with sharks and other primitive types 



is ambiguous, because various ideas are associated with these words " high " or 



Proceedings National Museum, Vol. XIV — No. 845. 



107 



