COMMON EARTHWORM. 209 



Bell, A. N. H. (5) xvi. 1885. Regeneration in marine Chaetopoda. De Quatrefages, 

 A. Sc. N. (3) ii. 1844, p. 100 ; Claparede, Anne"lides du Golfe de Naples, 1868, 

 p. 30. For lit. of subject in general, see Milne Edwards, Le9ons sur la Physiologic 

 et 1'Anatomie compared, Paris, viii. 1863, p. 301 et seqq. ; or Fraisse, Die Re- 

 generation von Geweben und Organen bei der Wirbelthieren, Cassel und Berlin, 

 1885 ; cf. Horst, Z. A. ix. 1886. 



41. COMMON EARTHWORM {Lumbricus terrestris, s. Agricold), 



Dissected so as to show its nervous system. 



THE integument has been divided down the middle dorsal line and 

 fastened out on either side. The entire digestive tract with the exception 

 of the buccal cavity, most of the nephridia or excretory organs, and the 

 septa dividing the body into compartments, have been removed. Of the 

 reproductive organs only the spermathecae or receptacula seminis, two 

 globular white sacs, have been left in situ on the right side. They open re- 

 spectively between the ninth to tenth and tenth to eleventh somites, on a 

 level with the dorsal row of setae. The two lobes, making up the supra- 

 oesophageal or cerebral ganglia, are pyriform, and have their broader ends 

 apposed to each othej in the middle line. A thick nerve passes off from 

 each of their outer or narrower ends. It bifurcates, and ends in a plexus 

 in the prostomium, on which are situate numerous sense-bodies. A right 

 and left oesophageal commissure surround the passage from the buccal cavity 

 to the pharynx and connect the supra-oesophageal to the first ganglion of 

 the ventral nerve-cord. This cord extends to the posterior extremity of 

 the body. It takes the shape of a thick band in which ganglionic enlarge- 

 ments are recognizable with difficulty for a space corresponding with that 

 occupied by the pharynx, oesophagus and reproductive organs. Posteriorly 

 to the fifteenth somite it becomes more slender, and the ganglia more 

 distinct. Finally, for a length nearly equal to the posterior half of the 

 animal, it becomes thicker and moniliform, the ganglia being plainly marked 

 but closely apposed. The terminal ganglion is, contrary to what is seen in 

 some Vermes and many Arthropoda, smaller than those which precede it. 



The two rows of paired setae are well seen on each side in most of the 

 somites. The enlarged inner copulatory setae of the two somites, fifth and 

 sixth in order anteriorly to the clitellum, as well as of the clitellum itself, 

 can be readily distinguished. In the interval between each inner row of 

 setae and the nerve-cord in the fifteen anterior somites, a longitudinal mus- 

 cular fascicle is seen passing forwards. It is inserted on the outer ends of 

 the supra-oesophageal ganglia and on the commissures, and acts as a re- 

 tractor muscle to these parts. Between the two rows of setae of the eighth 



p 



