146 FISHES CHAP. 
only partially illustrated by the presence or absence of biting 
jaws. 
In a general and popular sense the Cyclostomata are usually 
regarded as “ Fishes,” but this usage rests on no better founda- 
tion than a certain agreement between the Cyclostomata and the 
true Fishes in outward form and habits, and in their method 
of respiration by gills. On the other hand, it has been 
maintained that the distinctive features of the Cyclostomata 
are of sufficient importance not merely to separate them 
from the true Fishes, but possibly even (as is to some extent 
expressed by the use of the terms Agnathostomata and Gnatho- 
stomata) to warrant their elevation to a group equal in taxonomic 
value to all the remaining living Craniata taken collectively. 
The organisms included in the Cyclostomata, the Lampreys, and 
especially the Hag-Fishes, exhibit in many respects an extremely 
low grade of Craniate structure; but how far the simpheity or 
archaic nature of some of their organs is primitive, or has been 
acquired through degeneration, it is difficult, and is sometimes 
impossible, to determine with any degree of satisfaction. In 
other respects, such as the presence of a rasping “ tongue,” it 
is obvious that the Cyclostomata have attained a high degree 
of specialisation. As one of several illustrations which might 
be given of difficulties of this kind, it may be mentioned that 
it is by no means certain that the Cyclostomata are not the 
degenerate descendants of primitive but now extinct Gnatho- 
stomata. At all events the presence of paired cartilages in 
the skull of the Lamprey, which, with some show of reason, 
may be regarded as representatives of the primitive upper 
and lower jaws of the latter group, would seem to suggest this 
conclusion. If this be correct, we must regard the formation 
of a suctorial buccal funnel, with its complex system of sup- 
porting cartilages—one of the most striking features in the 
structure of this animal—as a secondary and adaptive special- 
isation of a mouth originally provided with biting jaws. But 
in spite of such difficulties there can be no question that the 
Cyclostomata are the most primitive of all existing Craniates, 
and so far differ from the true Fishes and from all other classes 
of Craniate animals, that their inclusion in a class by themselves 
is the least that can be done to give graphic expression to their 
isolated position, even if we do not fully accept the dictum of 
