SUB-KINGDOM I.— VERTEBRATA. 



CLASS VI.—CYGLOSTOMATA. 

 By R. Lydekkee, B.A., F.R.S., V.P.G.S., &c. 



LAMPREYS AND HAG-FISHES. 



As indicated by the name hag-fishes, which is popularly applied to the mem- 

 bers of one of the two families, the animals now to be considered are 

 commonly regarded as fishes. Popular natural history and scientific zoology 

 take, however, very different estimates of the mutual relationships of or- 

 ganised beings ; and zoologists are now pretty well agreed that, although 

 they were formerly admitted within its limits, lampreys and hag-fishes have 

 no real claim to be included in the class Pisces. Consequently, they are now 

 regarded as forming a class by themselves — the Cyclostomata. This class 

 includes the last group of animals now termed true Vertebrata; the lancelets, 

 which were likewise formerly regarded as fishes, being now considered as the 

 highest of the Protochordata or semi-Vertebrates. The essential features by 

 which the members of the lamprey group are broadly distinguished from the 

 higher classes of the Vertebrates are the complete absence of jaws, the single 

 — instead of double — aperture of the nostrils, and the rasping tongue. 

 Round-mouths — the English equivalent of Cyclostomata — is a name which 

 well expresses the first of those three distinctive features ; but, in allusion 

 to the second characteristic, the alternative name of Monorhina has likewise 

 been proposed for the class. Whichever name may be selected, there can be 

 no doubt as to the right of the lamprey group to constitute a class by itself. 

 Had we these living forms alone to deal with, there would be no need to 

 sub-divide the class into groups of higher rank than families. It happens, 

 however, that there are certain extinct, somewhat fish-like creatures, which 

 there is considerable reason for regarding as more or less distantly related 

 to the modern lampreys. Consequently, it is necessary to make the latter 

 into a sub-class by themselves ; and for this sub-class the name of Marsipo- 

 branchii has been selected. This name refers to the circumstance that in 

 the lampreys and hag-fishes the gills form a series of pouches on the sides of 

 the neck, on which they usually open by round apertures coincident in 

 number with the pouches. Either six or seven is the number of pairs of 

 gill-pouches, in which there are no supporting gill-arches. The sucking- 

 mouth, which occupies the extremity of the muzzle, is of circular form, with 

 its more or less lip-like margin sustained by a system of internal cartilages. 

 The skin of the body is soft, and unprovided with scales ; but the upper and 

 lower surfaces of its hinder half bear well-developed fins in the middle line, 

 which are supported by vertical rays of cartilage. Paired fins — correspond- 

 ing to those of fishes and representing the limbs of the higher Vertebrates — 

 are, however, conspicuous by their absence ; this lack of limbs forming, in- 

 deed, a characteristic of the class rather than a feature of the sub-class. 

 Internally, a lamprey lacks all traces of ribs, and the vertebral column is 

 either represented by the primitive notochord alone, or by that structure 

 surrounded by a series of calcareous rings. In reality, therefore, a lamprey 



