246 ELEMENTARY BIOLOGY. [CHAP. 



The nervous system is in an exceedingly simple and in- 

 teresting condition. It consists of two pear-shaped supra- 

 cesophageal ganglia which abut together in the middle line 

 in front ; from these there arise two commissures which, 

 like those of the Crayfish, run longitudinally side by side 

 for the whole ventral surface. These commissures are 

 somewhat enlarged for each somite, and there arise from 

 them a metamerically repeated series of nerve trunks. The 

 supra-oesophageal ganglia distribute fibres to the buccal 

 sac and to the anterior end of the body; the latter termi- 

 nate in modified sensiferous cells borne upon the first seg- 

 ment, which constitute all that the worm has in any way 

 representative of sense organs. One of the most important 

 facts concerning the nervous system of this animal, is the 

 disposition of the nerve-cells. These, instead of being re- 

 stricted to the ganglia, as they are in the Crayfish, are 

 regularly diffused throughout the entire axis — gangliform 

 enlargements and longitudinal commissures alike. The 

 sheath of the nervous axis is remarkable in being muscular, 

 and especially as concerns the existence of a dorsal ?ieuro- 

 chord made up of three longitudinally disposed '■'■tubular 

 fibres^ These fibres are highly clastic, and the muscular 

 sheath forms a most efficient protective apparatus. 



The proper reproductive organs are restricted to seven of 

 the anterior segments, and will be found described on p. 268. 

 They are somewhat complicated, chiefly owing to the con- 

 ditions of maturation of the seminal vesicles ; but the actual 

 genital glands, though exceedingly small, are well defined. 

 The ripe ovum consists of a round nucleated cell contain- 

 ing a moderate food-yolk, and invested in a vitelline mem- 

 brane. The sperm-producing cells undergo changes which 

 result in the formation of a number of filiform spermatozoa, 

 each with an elongated nucleus-bearing "//t\7^/;" and the 



