LUMBRICUS TERRESTRIS 



171 



and pulled, as it often is under these circumstances by a thrush, 

 either the chaetae, or the soil, or the worm, must break, and 

 usually it is the worm. 



The co-ordination of the contraction waves is carried out in 

 part mechanically because the stretching of the muscles causes 

 them to contract, and in part by the nervous system. The con- 

 traction of circular or longitudinal muscles in a somite 

 causes, through an impulse sent along the ventral cord, the 

 contraction of the corresponding muscles in the somite behind. 

 Further, in each somite the contraction of the circular muscu- 

 lature sends, by afferent and efferent fibres, through the 



^p.v.s: 



I 

 int. 



1Z^11\ 20 19 18; 17 1615^1413 12/11 /lO 9 ,' 8 7 



giz. 



cr. 



p.v.s. 



a. V. s. 



oes. 



ph. 



•\ 



3 \ 2 



v.n.c'. 



Fig. 120.— a diagram of a longitudinal section of an earthworm, 



a.v.s.. Anterior vesicula seminalis; a.v.s'., posterior lateral horn of the same overhanging the oesophagus; 

 cr., crop; giz.. gizzard : int., intestine; m., mouth ; oes., oesophagus ; p.v.s., posterior vesicula seminalis; 

 p.v.s'.. horn of the same overhanging the oesophagus ; per., peristomium ; ph., pharynx ; pr., prosto- 

 mium ; sup.ph.g., suprapharyngeal ganglion ; ty., typhlosole ; v.n.c, ventral nerve cord ; 1-23, 

 segments. The blood vessels are omitted. 



ganglion, impulses which relax the longitudinal musculature ; 

 and similarly the longitudinal muscles in contracting relax the 

 circular. Rapid contraction is co-ordinated by nervous impulses 

 which travel in the giant fibres, forwards in the laterals, and from 

 head to tail in the median fibre. 



SENSE ORGANS 



An earthworm has no well-developed organs of sense, but 

 certain of the columnar cells of the epidermis are rod-shaped 

 and prolonged at their inner ends into fibres, which run in the 

 nervous system. These are sense cells, and in the forepart of the 

 body some of them are collected into groups, which are rudimentary 

 sense organs. There are also sense cells which contain a refractive 



