82 VERTEBRATES WITHOUT JAWS iv. 2- 



that the receptors reveal. The swimming process, by the passage 

 of waves down the body, is itself perfected by improvements in the 

 shape of the fish, allowing rapid movements and turns. Besides the 

 median fins there develop lateral paired ones, serving at first a 

 stabilizing and steering function and then converted, when the land 

 animals arose, into organs of locomotion on the ground or in the air 

 and finally, in the shape of the hands, into a means of altering the 

 environment to suit the individual. 



The brain itself, at first mostly devoted to the details of sensory 

 and motor function, comes increasingly to preside, as it were, over 

 all the bodily functions, and to give to the vertebrates the 'drive' that 

 is one of their most characteristic features. The skull is developed as 

 a skeletal thickening around the brain, probably at first mainly for 

 protection, but later serving for the attachment of elaborate muscle 

 systems. The study of vertebrates is especially identified with study 

 of the skull, because in so many fossils this is the only organ preserved. 



The food of the earliest vertebrates was collected by ciliary action, 

 but this habit has long been abandoned and only in rare cases today 

 does the food consist of minute organisms. The pharynx of most 

 vertebrates is small, there are relatively few gill-slits and these are 

 respiratory. In all except the most ancient forms the more anterior of 

 the arches between the gills became modified to form jaws, serving not 

 only to seize and hold the food but also to 'manipulate' the environ- 

 ment. 



The blood system shows two of the most characteristic vertebrate 

 features, namely, the presence of a heart that has at least three 

 chambers and thus provides a rapid circulation, and of haemoglobin 

 within corpuscles, serving to carry large amounts of oxygen to the 

 tissues. The efficiency of this system must have been a major factor 

 in producing the dominance of the vertebrate animals. In the air- 

 breathing forms, and especially the warm-blooded birds and mam- 

 mals, the respiratory and circulatory systems allow the expenditure of 

 great amounts of energy per unit mass of animal, so that quite extra- 

 vagant devices can be used, allowing survival under conditions that 

 would otherwise not support life. 



The excretory system is based on a plan quite different from that 

 of amphioxus. It consists of mesodermal funnels, leading primarily 

 from the coelom to the exterior. It may be that this type of kidney 

 arose in connexion with the abandoning of the sea for fresh water. 

 Probably all but the earliest vertebrates have passed through a fresh- 

 water stage, and it is significant that all except Myxine have less salt in 



