48 THE BIOLOGY OF AN ANIMAL, 



In the median dorsal line of eacli somite (excepting the first 

 two or three) is a minute pore (the dorsal j^ore) which perfo- 

 rates the body-wall and thus places the coelom in connection 

 with the exterior."^ Other pores that pass through the body- 

 wall into the cayities of yarious organs will be described fur- 

 ther on. 



Organs of the Animal Body. Systems of Organs. The body of 

 the earthworm consists essentially of protoplasm, and in order that 

 so large a mass of liying matter may continue to exist and carry 

 on the ordinary life of an earthworm it must be able to obtain 

 a sufficient supply of food; to digest and absorb it, and dis- 

 tribute it to all parts of the body ; to build up new protoj^lasm 

 and remoye waste. It must be sensitiye to external and internal 

 influences ; capable of motion and locomotion. Aboye all, each 

 part must act with reference to, and in harmony with, eyery 

 other part, so that the organism may not be merely an aggregate 

 of organs, but one body acting as a unit or a whole. 



T\\Q^Q functions are fulfilled by the organs, respectiyely, of 



ALIMENTATION, DIGESTION, ABSORPTION, CIRCULATION, EXCRETION, 



SENSATION, MOTION, and COORDINATION. All of tlicsc minister to 

 the welfare of the indiyidual. The REPRODucTiyE function, on 

 the other hand, and its corresponding organs, serye to perpet- 

 uate the species, thus ministering rather to the race than to the 

 indiyidual. 



Sets of organs deyoted to the same function constitute syS' 

 terns I as the alimeiitary system^ the circulatory system^ etc. 

 Those wdiich are more immediately concerned with the income 

 and outgo of matter — namely, the alimentary, digestiye, absorp- 

 tiye, circulatory, and excretory systems — are sometimes called the 

 "vegetative systems or systems of nutrition '^ while those which 

 haye to do more immediately with the relation of the body to 

 its enyironment, rather than the indiyidual itself, are called syS' 

 terns of relation. Examples of the latter are the systems of 

 organs of support, motion (including locomotion), sensation, and 

 coordination ; and eyen the reproductiye system, as relating chiefly 

 to other indiyiduals, finds a place here. 



* If living worms be irritated they will often extrude a milky fluid from, 

 these pores, but the use of the latter is not well understood. 



