IV. VERTEBRATA: CYCLOSTOMATA. 491 



apparatus of the embryo, and in the mammals it develops the placenta, 

 by which nourishment as well is conveyed to the young. Yolk sac, am- 

 nion, and allantois are enveloped in a common coat, the serosa. 



Aristotle recognized four divisions of vertebrates, and these were retained 

 by Linne and Cuvier under the names Pisces, Reptilia or Amphibia, Aves, and 

 Mammalia. Blainville divided the second of these into two classes, retaining 

 the name Reptilia for one, Amphibia for the other. Milne Edwards showed 

 that this division corresponded with one between the higher and lower groups, 

 the amniote and the anamniote divisions. Later Haeckel separated the Cyclo- 

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

 close resemblances between reptiles and birds, uniting them as Sauropsida. 

 Another convenient division contrasts the fishes with all other forms, the Tetra- 

 poda, so called because they have legs rather than fins. 



Series I. Ichthyopsida (Anamnia, 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 of lamprey eels and 

 slime or hag fishes. 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 (fig. 532, /) is not divided into utriculus and 

 sacculus, and it has but one or two semicircular canals, hut always two 

 ampullse. The skin (ilg. 27) consists of corium and a stratified epidermis. 



The cyclostomes are distinguished from the true fishes by the lack of 

 a vertebral column. The a.xial skeleton of the trunk consists either of 

 the notochord alone or of it and small cartilaginous arcs, representing 

 neural arches and intercalaria. A cranium and a basket-like gill skeleton 

 are present, but so different are these from those of other vertebrates that 

 homologies are diflicult. The absence of paired fins is important. The 

 median fins are supported by horny threads alone, and the cartilaginous 

 appendicular skeleton — alone of importance — is entirely wanting. Then 

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

 teeth, arranged in circles in the mouth of the lamprey (fig. 542), and the 

 fewer teeth of the myxinoids, are purely epidermal products and cannot 

 be compared with the teeth of other vertebrates. 



The name Cyclostomata refers to the circular mouth, which rests on 

 the important fact that the jaws are absent (Agnatha) or extremely 



