iv. 4 AGNATHA 83 



their blood than there is in sea water. Elaborate devices for regulation 

 of osmotic pressure have been developed, and the mesodermal kidneys 

 play a large part in this regulation. 



This outline only gives a few suggestive features of vertebrate 

 organization. The details differ bewilderingly in the different types 

 and it is our business now to survey them. In the earliest forms the 

 more special mechanisms are absent or at least function only crudely, 

 and passing through the vertebrate series we find more and more 

 devices adopted, along with more and more delicate co-ordination 

 between the various parts, culminating in the extremely highly 

 centralized control of almost every aspect of life that is exercised by 

 the mammalian cerebral cortex. 



3. Agnatha 



The earliest vertebrates, while showing most of the characteristic 

 features of the group, differ from the rest in the absence of jaws and 

 are therefore grouped together in a superclass Agnatha, distinguished 

 from the remaining vertebrates, which have jaws, and are therefore 

 called Gnathostomata. The only living agnathous animals are the 

 Cyclostomata, lampreys and hag-fishes, but the first vertebrates to 

 appear in the fossil series, mostly heavily armoured and hence known 

 as 'ostracoderms', found in Silurian and Devonian strata, also show 

 the agnathous condition, and have some other features in common 

 with the Cyclostomata. This group of agnathous vertebrates shows 

 some interesting experimentation in methods of feeding, before the 

 jaw-method became adopted. The modern cyclostomes are parasites 

 or scavengers, in the adult state, but as larvae the lampreys still feed 

 on microscopic material, using an endostyle resembling that of 

 amphioxus in many ways, but making use of muscular contraction 

 rather than ciliary action to produce a feeding current. The methods 

 of feeding of the Devonian forms are not known for certain, but 

 probably included shovelling detritus from the bottom. 



The Cyclostomata are therefore worth special study as likely to 

 show us some of the characteristics possessed by the earliest vertebrate 

 populations. 



4. Lampreys 



The most familiar cyclostomes are the lampreys, of which there are 

 various sorts found in the temperate zones of both hemispheres. All 

 lampreys have a life-history that includes two distinct stages: the 

 ammocoete larva lives in fresh water, buried in the mud, and is 



