168 The Cyclostomes, or Lampreys 
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, wth 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 modern 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 fossil Cyclostomes are. 
known. The strange forms called Conodontes, thought for a 
time to be teeth of lampreys, are probably teeth of worms, or 
perhaps appendages of Trilobites. The singular fossil, Palgo- 
spondylus, once supposed to be a lamprey, it is certain belongs 
to some higher order. 
Orders of Cyclostomes.—The known Cyclostomes are natu- 
rally divided into two orders, the Hyperotreta, or hagfishes, and 
the Hyperoartia, or lampreys. These two orders are very dis- 
tinct from each other. While the two groups agree in the general 
form of the body, they differ in almost every detail, and there is 
much pertinence in Lankester’s suggestions that each should 
stand as a separate class. The ancestral forms of each, as well 
as the intervening types if such ever existed, are left unrecorded 
in the rocks. 
The Hyperotreta, or Hagfishes.—The Hyperotreta (ixCpoa, pal- 
