IV. VERTEBRATA: CTCLOSTOMATA. 555 



with one between the higher and lower groups, the amniote and the anam- 

 niote divisions. Later Haeckel divided the fishes, separating the Cyclo- 

 stomes from the others as a distinct class, while Huxley pointed out the 

 close resemblances between the reptiles and birds, grouping them as 

 Sauropsida. Another division of convenience but not of much systematic 

 importance contrasts the fishes with all other forms, the Tetrapoda, so 

 called from the possession of legs rather than fins. 



SERIES I. ICHTHYOPSIDA (ANAMXIA, ANALLANTOIDA). 



Vertebrates respiring for a time or throughout life by means 

 of gills ; neither amnion nor allantois present in the embryo. 



Class I. Cyclostomata (Marsipobranchii, Agnatha). 



The class of Cyclostomes contains but few species, among 

 which the lamprey eels and the slime or hag fishes are best known. 

 In shape they are eel-like. They are distinctly vertebrate in the 

 possession of large liver and nephridia; of a muscular heart with 

 auricle and ventricle, lying in a pericardium; olfactory lobes, 

 epiphysis and hypophysis, and the higher sense organs. In the 

 brain, cerebrum and cerebellum are not so prominent as are the 

 optic lobes and medulla. The inner ear is not divided into utric- 

 ulus and sacculus, and it has but one or two semicircular canals, 

 but always two ampullae. The skin (fig. 26) consists of derma 

 and a stratified epidermis. 



The cyclostomes are distinguished from the true fishes by the 

 lack of a vertebral column. The axial skeleton of the trunk consists 

 either of the notochord alone or of it and small neural arches. A 

 cranium and a basket-like gill skeleton are present, but so different 

 are these from those of other vertebrates that homologies are dif- 

 ficult. The absence of paired fins is important. Since the median 

 fins are supported by horny threads alone, the cartilaginous appen- 

 dicular skeleton alone of importance is entirely wanting. Then 

 the skin lacks scales, and the mouth, true dentine teeth, for the 

 pointed brown teeth arranged in circles in the mouth of the lam- 

 prey (fig. 584), and the fewer teeth of the myxinoids, are purely 

 epidermal products and cannot be compared with the teeth of 

 other vertebrates. Other important differences have given rise to 

 names applied to the group. 



The name Cyclostomata refers to the circular mouth, an ex- 

 ternal feature, which, however, rests on the important fact that 

 the jaws are absent or extremely rudimentary, and do not close on 

 each other as do the jaws of other vertebrates. This cyclostome 

 condition is of value to the animals, as it aids them in sucking on 



