1 68 



EARTHWORMS. PHYLUM ANNELIDA 



sup ph.g 



in the ccElom below the gut, swelHng into a ganglion in each 

 somite The first of these ganglia is bilobed and is known as the 

 subpharyngeal ganglion. Nerves are given off to the prostomium 

 from the suprapharyngeal gangha, and to the first two somites 

 from the commissures, and the ventral cord gives off in each 

 somite three pairs of nerves which run upwards as girdles in the 

 body-wall, giving off branches as they go. The ahmentary canal 

 receives nerves from the circumpharyngeal commissures and 

 fibres from plexuses in the septa. Though the ventral cord 

 appears to be single, it is really double, and can be seen in trans- 

 verse sections to be rather imperfectly 

 divided into right and left halves by 

 connective tissue. Transverse sections 

 also show that the middle and upper 

 parts of the cord consist of fine, 

 chiefly longitudinal, nerve fibres, and 

 the lower and outer parts contain 

 nerve cells. Above the mass of fine 

 fibres are three longitudinal bundles 

 of such fibres, each bundle being 

 enclosed in a sheath and known as a 

 giant fibre. Nerve cells are more 

 numerous in, but not confined to, 

 the ganglia. The nerves consist of 

 afferent fibres, which start from sense 

 cells in the epidermis and muscles 



c.<»/i.c., Circumpharvngeal commissure ; n., ,-^. ^ -, • i x xt_ 



nerves; ph., pharynx cut through; (Fig. Iig) aud Carry impulSCS tO thC 

 Sep., septa ; subph.g., subpharyngeal . , , • i_ • t_ a.i^ 



ganglia ; sup.ph.g., suprapharyngeal Central uervous system, lu which tuey 

 f^^'^mi't£;^^"^''^°^''^"^^^''°'' ' end as bunches of efferent fibres, 



which start from nerve cells in the 

 ganglia and end against muscle and other cells, to which they 

 convey impulses and also of fibres which join nerve nets in the 

 skin, muscles, and septa. 



MOVEMENT 



On the surface, worms move by means of a peristaltic con- 

 traction of the muscles of the body-wall. Two series of waves of 

 contraction, one of the longitudinal and one of the circular muscles, 

 pass along the body from the anterior end ; the waves of the 

 two series are out of phase with each other, contraction in one 

 set of muscles in a particular segment being accompanied by 



Fig. 1 1 8. — A diagram of the 

 forepart of the nervous sys- 

 tem of the earthworm. 



