ANNULATA. 225 



lacunae of the coelom may penetrate. In some forms there 

 are special thin places and outpocketings of the body-wall 

 (branchiae) by which the exchange of gases is facilitated. 

 These are characteristic of the Polychaeta especially (Figs. 

 105, 106). 



263. Circulation. In some of the simplest worms there 

 are no special blood vessels. The coelomic spaces contain a 

 fluid, which possesses corpuscles and is moved by the general 

 body contractions. In the typical condition there are two or 

 more 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. 



264. Excretion takes place by means of the segment al 

 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. 33) : (i) a ciliated funnel, com- 

 municating 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. 



265. Nervous System. The " central " nervous 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. 100). The brain and 



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