nerves which supply the various parts and organs with- 

 in that metamere. 



On the opposite, or haemal side of the body, run- 

 ning parallel with the alimentary canal and spinal cord, 

 is the great vascular distributing canal, or primitive 

 heart tube. Its paired blood vessels, with the paired 

 nerves, form approximately parallel, hoop-like, con- 

 ducting channels around the alimentary canal between 

 it and the outer surface of the body. 



We recognize in this type of animals, a wholly new 

 architectural unit, the metamere, each one approxi- 

 mately complete in itself. The whole body primarily 

 consists of a chain of similar metameres, organically 

 united, each link in the chain subject to the local con- 

 ditions of its bodily position, and ultimately express- 

 ing them in the varying development of its component 

 parts. 



While growth may take place at any point in this 

 system, the system is chiefly remarkable for the fact that 

 organs around the head end are the first to be formed; 

 and each new metamere makes its appearance as a dis- 

 tinct entity at the tail end, just behind the one pre- 

 viously formed. No basic alteration of this order of 

 precedence is known to occur. 



Hence the body always consists of a graded, linear 

 series of metameres, in one, two, three, order. Whether 

 there be two or three, or several score, the one nearer 

 the head end is the older, the one behind it, the 

 younger; and each metamere retains its serial identity 

 throughout life. 



On the other hand, the neural axis is always more 

 precocious than the haemal, so that differential growth 



