The Cyclostomes, or Lampreys 487 



the detailed homology of its different parts offers considerable 

 uncertainty. The heart is modified to form two pulsating 

 cavities, auricle and ventricle. The folds of the dorsal and 

 anal fins are distinct, supported by slender rays. 



The mouth is a roundish disk, with rasping teeth over its 

 surface and with sharper and stronger teeth on the tongue. 

 The intestine is straight and simple. The kidney is represented 

 by a highly primitive pronephros and no trace exists of an 

 air-bladder or lung. The skin is smooth and naked, some- 

 times secreting an excessive quantity of slime. 



From the true fishes the Cyclostomes differ in the total 

 absence cf limbs and of shoulder and pelvic girdles, as well as 

 of jaws. It has been thought by some writers that the limbs 

 were ancestrally present and lost through degeneration, as in 

 the eels. Dr. Ayers, following Huxley, finds evidence of the 

 ancestral existence of a lower jaw. The majority of observers, 

 however, regard the absence of limbs and jaws in Cyclo- 

 stomes as a primitive character, although numerous other features 

 of the modem hagfish and lamprey may have resulted from 

 degeneration. There is no clear evidence that the class of 

 Cyclostomes, as now known to us, has any great antiquity, and 

 its members may be all degenerate offshoots from types of 

 greater complexity of structure. 



Supposed Extinct Cyclostomes. — No species belonging to the class 

 of Cyclostomes has been found fossil. We may reason theoretic- 

 ally that the earliest fish-like forms were acraniate or lancelet- 

 like, and that lamprey-like forms would naturally follow these, 

 but this view cannot be substantiated from the fossils. Lance- 

 lets have no hard parts whatever, and could probably leave 

 no trace in any sedimentary deposit. The lampreys stand 

 between lancelets and sharks. Their teeth and fins at least might 

 occasionally be preserved in the rocks, but no structures cer- 

 tainly known to be such have yet been recognized. It is how- 

 ever reasonably certain that the modern lamprey and hagfish 

 are descendants, doubtless degraded and otherwise modified from 

 species which filled the gap between the earliest chordate animals 

 and the jaw-bearing sharks. 



Conodontes. — Certain structures found as fossils have been 

 from time to time regarded as Cyclostomes, but in all such 



