64 THE BIOLOGY OF AN ANIMAL. 



the reproductive elements (p. 74), and assist in the fertilization of the Qg^ 

 (p. 74). Their action, like that of the muscle-fibres, is doubtless due to the 

 property of contractility., the protoplasm alternately contracting on opposite 

 sides of the cilium and thus causing its whiplike action. 



White Blood-corpuscles. Amoeboid Cells. Lymph-cells. Phagocytes, 

 Besides muscle-cells and ciliated cells there is a third variety which display 

 contractility and movement. These are the coelomic corpuscles referred to 

 above (p. 53). Until recently their function was wholly unknown, but it 

 is now generally believed that they are the scavengers of the body, devour- 

 ing the dead tissues or foreign bodies which invade the organism. Whether 

 they also attack and devour living parasites such as Qregarina and Bacteria 

 is not yet fully determined. They move their parts much as Amoebae do, 

 engulfing particles about them by a kind of flux. 



Nervous System. Organs of Coordination. 



Introduction. The general office of the nervous system of 

 organs is to regnlate and coordinate the actions of all the other 

 parts in such wise that these actions shall form an harmonious 

 and orderly whole. Through ner\'ous organs the worm receives 

 from the environment impressions which pass inwards through 

 the nerves as sensory or afferent unpulses, to the nervous centres ; 

 and through other nervous organs impulses (efferent or motor) 

 pass outwards from the centres to the various parts so as to 

 arouse, modify, or suspend their activities. Tlius tlie animal is 

 enabled to call forth movements resulting in the two kinds of 

 adjustments referred to on p. 6i^, viz., {a) adjustments of the 

 body as a whole to changes in the environment (e. g. , the with- 

 drawal of the earthworm into its burrow at the approach of day) ; 

 and ih) adjustments between the parts of the body itself, so that 

 a change in one part may call forth answering changes in other 

 parts (e.g., the increased supply of blood to tlie alimentary canal 

 during digestion, or vigorous movements of the fore end of the 

 body when the hind end is irritated). 



These functions are always performed by one or more nerve- 

 cells^ which give off long slender branches known as nerve-fibres 

 usually gathered together in bundles, the nerves^ extending into 

 all parts of the body. In all higher animals the main bulk of 

 the nerve- cells are aggregated in definite bodies known as 

 ganglia.^ out of which, into which, or through which, the nerves 

 proceed ; and as a matter of convenience it is customary to desig- 

 nate the most important of these ganglia collectively as the cen- 



