40 ESSENTIALS OF BIOLOGY 



contradistinction to the outer exo-skeleton of the arthropods. 

 There are a number of " fore-and-aft " segments in the body of 

 a vertebrate, even though the body may appear to be single and 

 undivided. This fundamental segmentation is indicated (for 

 instances) in the jointing of the vertebral column and in the 

 repetitional arrangement of the nerves coming from the brain 

 and spinal cord. 



There are always four limbs : 



Two pectoral and two pelvic fins in Fishes ; 



Two fore-limbs and two hind- limbs in Amphibians and 

 Reptiles ; 



Two fore-limbs (wings) and two hind-limbs (legs) in Birds ; 



Two fore-limbs and two hind-limbs in quadrupeds ; 



Two arms and two legs in man and other Primates. 

 (But limbs may be quite absent in the adult stages (in some 

 snakes, etc.). 



There is a skull in most chordates and this is essentially a 

 bony case enclosing the brain and partially enclosing the great 

 head sense organs. There is a spinal canal in the vertebral column 

 and this encloses the spinal cord. The whole central nervous 

 system, brain and cord, is dorsal to the alimentary canal. (It 

 is ventral to the latter in the worms and arthropods.) And so on. 



In the above treatment of animal types we have, in the main, 

 followed the lines of a rational classification of animals (see 

 Section 94). But only slightly, since even a conspectus of the 

 structural features of such a classification would be a lengthy 

 matter. It would take account of extinct and degenerate species 

 and larger groups ; it would weight all those structural characters 

 that are indicative of phylogenetic relationships (Section 97) and 

 it would tend to neglect such superficial (though perhaps very 

 striking) structural features as indicate changed habits, some 

 adaptations, degeneracy, parasitism, the assumption of colonial 

 habits, etc. The basis of a rational classification is (so to speak) 

 a '* purged " morphology and we cannot (and need not) deal with 

 it here. We are concerned with the existing appearances and 

 activities of animals and, for the present, not at all with their 

 genetic relations and evolution. Thus we look upon types of 

 living things, categories into which we include the forms that are 

 like each other (in general ways) in their modes of life, general 

 bodily structures and physiologies. 



