84 ESSENTIALS OF ZOOLOGY 



possession of the primary schizocoel. The mechanical aid of 

 the coelom is called in, and the nephrostome discharges some 

 of the waste products brought by the coelomic amoeboid 

 cells. 



Nervous System. The brain is formed of two cerebral 

 ganglia which lie just above the buccal sac. The brain gives 

 off a large number of fibres to the prostomium. It gives off 

 also the two cords which encircle the alimentary canal in front 

 of the pharynx, the circumoesophageal commissures ; these 

 meet below the alimentary canal to form the anterior end of 

 the ventral nerve cord, and this extends to the posterior end 

 of the worm. The cord expands into a ganglion in each 

 segment. Its double nature and origin are indicated by the 

 connective tissue which occupies the middle line of a section. 

 The nerve cells are diffused throughout the cord and give off 

 branches or nerve fibres which leave the cord in bundles 

 called nerves, especially in the segments, and are distributed 

 in serial repetition in each segment to the organs and to the 

 muscles and the skin. The cells are called neurons, and the 

 branches of the cells or nerve fibres axons. On each side of 

 the ventral nerve cord large axons, which have been called 

 giant fibres, are seen in section. 



There are no sense organs eyes, ears, and nose but the 

 worm is evidently sensible ofxlay and night and of disturbances 

 communicated through the soil. 



Reproductive Organs. The earthworm is hermaphrodite. 

 The sex cells are developed at special places on the coelomic 

 wall. There are two pairs of testes, branched outgrowths of 

 the front septa of segments 10 and 11. They project into the 

 coelom, and the male cells in an immature condition are 

 liberated into the coelom. They are taken up by folds of the 

 septal walls of these segments, and the sacs formed project 

 on each side of the oesophagus as conspicuous white bags. 

 These are the seminal vesicles, and within these diverticula of 

 the coelom the male cells undergo division into balls of cells. 

 The cells then develop flagella, and are thus converted into 

 spermatozoa. The spermatozoa are passed to the exterior by 

 special ducts, the vasa deferentia. From the posterior walls 

 of the tenth and eleventh segments two pairs of funnels are 



