246 ELEMENTARY BIOLOGY. [CHAP. 



sists of a watery plasma in which are suspended exceedingly 

 minute transparent non-nucleated corpuscles. These are 

 somewhat variable in shape and size, being usually about 

 the ^<yVo tn f an mcn * n l en gth ; their structural features 

 agree closely with those of the nuclei of the epithelial lining 

 of the vessels in which they circulate. 



The feature which most clearly distinguishes the red- 

 blood fluid of the worm from that of the Frog, is that its 

 colouring matter is diffused through the plasma and in no 

 way related to the corpuscles. 



The nervous system is in an exceedingly simple and in- 

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

 otsophageal 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-cesophageal 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 neuro- 

 chord made up of three longitudinally disposed "tubular 

 fibres" presenting, micro-chemically, leading features in 

 common with medullated nerve fibres. 



The proper reproductive organs are restricted to seven of 



