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 simplicity 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 iiot fully accept the dictum of 



