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 pore) 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 cavities of various 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 living 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 protoplasm 

 and remove waste. It must be sensitive to external and internal 

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

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

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

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



These functions are fulfilled by the ORGANS, respectively, OF 



ALIMENTATION, DIGESTION, ABSORPTION, CIRCULATION, EXCRETION, 



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

 the welfare of the individual. The REPRODUCTIVE function, on 

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

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

 individual. 



Sets of organs devoted to the same function constitute sys- 

 tems ; as the alimentary system, the circulatory system, etc. 

 Those which are more immediately concerned with the income 

 and outgo of matter namely, the alimentary, digestive, absorp- 

 tive, circulatory, and excretory systems are sometimes called the 

 vegetative systems or systems of nutrition while those which 

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

 its environment, rather than the individual itself, are called sys- 

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

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

 coordination ; and even the reproductive system, as relating chiefly 

 to other individuals, 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. 



