232 ZOOLOGY 



longitudinal vessels, dorsal and ventral (or lateral) in position. 

 These are often connected by transverse loops in a few or many 

 segments of the body especially at the anterior and posterior 

 ends. The circum-intestinal loops are often contractile, and 

 the longitudinal vessels may show a wave of contraction passing 

 from one end to the other. Capillaries vary much in perfection 

 of development. The blood contains only white corpuscles. 

 There is hsemoglobin in the plasma. 



270. Excretion takes place by means of the segmental organs 

 or nephridia, of which there is usually one pair in each segment, 

 with the exception of some of the anterior segments. The 

 nephridium is a tubular structure consisting essentially of the 

 following portions (Fig. 35): (i) a ciliated funnel, communi- 

 cating with the coelom; (2) a tortuous glandular region; and (3) 

 an outlet through the body wall, often supplied with muscle 

 fibres. The nitrogenous waste products find their way into the 

 fluid of the coelom and thence into the nephridium, or directly 

 into the nephridium from blood capillaries which may occur in 

 its walls, and thus are finally eliminated upon the exterior of 

 the body. 



271. Nervous System and Behavior. — The "central" nerv- 

 ous system may be said to consist of three portions: (i) a mid- 

 ventral line of nerve fibres, and nerve cells which are diffusely 

 scattered or collected in ganglia, (2) a brain which is anterior 

 and dorsal to the pharynx, (3) a connective, or collar about the 

 pharynx connecting (i) and (2) (Fig. 102). The brain and 

 ventral cord may show distinct right and left lobes or may be 

 completely fused into a median mass. From the brain, nerves 

 pass to the head-parts. From each of the segmental portions 

 of the ventral chain nerves pass to the walls, viscera, etc. The 

 ventral cord frequently lies in a blood sinus which provides its 

 abundant nourishment (leeches). 



The sense organs occur very unequally in the group. The 

 Polychasta and the leeches are best supplied^ The integument 

 is generally sensitive tO'mechanical contact, to chemical stimuli 

 and to moisture. This sensitiveness is perhaps specially local- 

 ized in the tentacles, cirri, and more movable parts. Statocysts, 



