THE CYCLOSTOMATA. 383 



and haemal arches, and vertebral centra are not recognizable in the ostracoderms, 

 yet they make their appearance in the elasmobranchs, holocephali and teleostomi, 

 in each case apparently arising independently of the other. There is also a well- 

 marked tendency in all these three groups, or in their more remote descendants, 

 for the lateral eyes, olfactory, and auditory organs, to become greatly enlarged; 

 for the eyes and olfactory organs to take up a more lateral position ; for the paired 

 jaws to unite; the trunk and caudal segments to increase in number; the viscera 

 and anus to take up a more caudal position, and for the dental plates and branchial 

 chamber to disappear. 



At some time, probably not later than the Silurian period, at least three or four 

 well denned phyla grew out of the ostracoderms, as indicated in the accompanying 

 table. (Fig. 258.) The main line of ascent probably leads from the typical 

 ostracoderms, through the antiarcha and arthrodires, to the crossopterygians, 

 dipnoi and amphibia. Evolution along this line is steady, comparatively rapid, 

 and in every respect leads consistently upward to the first air-breathing land 

 vertebrates, the culminating metamorphosis depending on remote antecedent 

 changes in the dermal skeleton, appendages, air bladder, and heart. 



The remaining phyla stand quite apart from this main stem. They are 

 characterized by the breaking up of the dermal armor into minute plates (elasmo- 

 branchs) or by their absence altogether (the later holocephali); by the absence 

 of an air bladder, branchial chamber, and leg-like fins. At no time in their 

 history, so far as it is known, do they show any indications whatever of develop- 

 ing into air-breathing vertebrates. They do not possess the necessary anatomical 

 structures, and their evolution takes them into quite other directions. The 

 cyclostomes end in lampreys; the holocephali in chimaeras, and the elasmobranchs 

 in sharks and rays. We may characterize the several phyla arising from the 

 ostracoderms as follows: 



I. CYCLOSTOMATA. 



The cyclostomes may be regarded as one of the earliest off-shoots of the ostra- 

 coderms. We may consider their chief characteristics under three heads, namely 

 those derived from the ostracoderms, those gained, and those lost since their 

 separation from them. 



i. The cyclostomes retain the following organs derived from the ostraco- 

 derms: A median, practically unpaired, olfactory organ, merged with a persistent 

 hypophysis that opens on the dorsal surface of the head. An uncommonly large 

 and well developed parietal eye. The lateral eyes, on the contrary, are relatively 

 small, and are very late in acquiring a functional union with the superficial ecto- 

 derm. Hence, except for the insignificant visual power possessed by the parietal 

 eye, there is a long post-embryonic blind period corresponding to the permanent 

 blind period in some of the earlier ostracoderms. It is explained on the assump- 

 tion that the lateral eyes, during the ostracoderm stage in the phylogeny of the 



