292 ZOOLOGY FOR MEDICAL STUDENTS chap. 



members of the animal kingdom. The phylum is of particular human 

 interest and of special importance to the student of medicine, for it 

 includes Man, and the study of its more archaic members affords many 

 clues to the evolutionary history of the organs met with in the human 

 being. 



The Vertebrata are on the whole of relatively great size as compared 

 with the invertebrates, p,nd as they are of active habits this largeness of 

 size involves comparatively great complexity of structure. Amongst this 

 complexity certain features stand out as specially characteristic of the 

 typical vertebrate. 



The central nervous system is, as in the annelid or arthropod, con- 

 centrated into a longitudinal nerve-cord but this is characteristically 

 tubular, being perforated longitudinally by a central canal. A striking 

 physiological difference between the vertebrate and the annelid or arthro- 

 pod, and one which has had a profound influence on the evolution of 

 the phylum, is that the side of the body along which the nerve-cord runs 

 — the neural side — is uppermost in the normal position of the body 

 instead of being underneath, next the ground, as it is in a typical annelid 

 or arthropod. Consequently all these features of structure — and they 

 are very many — ^which are adaptations to the ordinary position of the 

 body, are in a sense reversed as compared with the condition in annelids 

 or arthropods. When the terms dorsal and ventral are used in regard 

 to a vertebrate they refer simply to the position of the body normal to 

 this phylum. The neural surface of a vertebrate is dorsal, that of an 

 annelid is ventral. 



The large size of the vertebrate and its activity of movement involve 

 as in the arthropod the presence of a well-developed skeleton and the 

 division of this into movable pieces, but whereas in the arthropod the 

 skeleton is an exoskeleton, developed completely external to the living 

 substance, in the vertebrate on the other hand it is an internal skeleton. 

 This is built up of three elements (i) the notochord — a longitudinal elastic 

 rod of cells split off from the neural (dorsal) surface of the alimentary 

 canal ; (2) blocks of the modified connective tissue known as gristle or 

 cartilage — in which the rounded cells are embedded in a stiff translucent 

 matrix possessing the chemical peculiarity that it gives rise to gelatine 

 when acted on by boiling water under pressure ; and (3) bone^ — also a 

 modified connective tissue in which the cells are branched and the matrix 

 heavily infiltrated with salts of calcium. 



The muscular system of the vertebrate consists for the most part — 

 in at least early stages of development — of longitudinal fibres arranged 

 on each side of the body in segmental blocks or myotomes. 



