384 THE VERTEBRATES. 



vertebrates, had for the first time been forced into the brain chamber by the in- 

 folding of the medullary plate, and had not completely regained their functional 

 relations with the outside world. 



There are, in the adults of some genera, three pairs of oral arches provided 

 with rudimentary appendages. The arches are comparable with the three 

 pairs in amphibian embryos, i.e., premaxillae, maxillae, and mandibles, and with 

 the three dental-plate arches of adult ostracoderms and arthrodires. (Fig. 175.) 

 The true mouth is not always circular, but may be a narrow, longitudinal slit. 

 The oral region may be surrounded by a wide papillate fold of ectoderm that 

 forms a shallow, circumoral antechamber, or pre-oral hood, comparable with 

 the membranous folds on the free anterior edges of the dorsal and ventral shields 

 of Bothriolepis. 



The thyroid gland, which represents a liver-like diverticulum comparable 

 with that on the haemal surface of the thoracic gut in arachnids (Figs. 43, 44, 

 181, 182, 308), is very large, reaching here its maximum development in primi- 

 tive vertebrates. The peribranchial chamber may be in part retained. 



2. The cyclostomes have lost, probably at a very early period, their an- 

 cestral dermal armor, including the jaw plates; also the cephalic swimming 

 appendages and lateral folds; and the tadpole-like form does not appear in any 

 phase of their development. 



3. The cyclostomes have failed to develop many important organs that have 

 appeared in the other descendants of the ostracoderms. There are no paired 

 pectoral or pelvic appendages; the notochord persists in a practically unmodified 

 condition; no traces of ring-like calcifications of its sheath appear, and only the 

 most diminutive neural and haemal spines are developed. The air bladder 

 and teeth of the vertebrate type are absent. 



The actual progress made by the cyclostomes since their separation from 

 the ostracoderms is therefore insignificant, the only noteworthy gain being in the 

 increased number of body segments, giving additional freedom and facility of 

 locomotion, and an imperfect adaptation to a parasitic mode of life. The cyclos- 

 tomes may therefore be regarded as very ancient animals, deriving their underlying 

 primitive characters from the ostracoderms, and owing the present simplicity of 

 their organization to a precocious senility that was never preceded by a vigorous, 

 creative youth. 



II. THE ELASMOBRANCHII AND HOLOCEPHALI. 



The Elasmobranchii. The advent of the elasmobranchs is clearly fore- 

 shadowed in the Silurian period by the appearance of the ccelolepidae with their 

 fish-like form and shagreen-like armor. The successive steps in the fragmentation 

 and final disappearance of the dermal armor are well shown in this branch of the 

 ostracoderms. The process begins in the cephalaspidas with the formation of well- 

 marked, but immovable polygonal areas, followed by the appearance of the 

 small free plates of Ateleaspis and Lasanius, and by the isolated dermal denticles 

 of Thelodus and the elasmobranchs, which finally disappear altogether in their 



