236 



METAZOAN PHYLA 



the vessels. Peristalsis is the passage of a series of rhythmic contrac- 

 tions along a vessel which advances the contents of that vessel in the 

 same direction as that in which the waves of contraction progress. In 

 metameres VII to XI are five pairs of dilated vessels, often called hearts 

 (Fig. 146), which connect the dorsal to the ventral blood vessels. They 

 are really dilated vessels exhibiting powerful peristaltic movements. 

 Valves in the dorsal vessel and hearts prevent back flow. The blood 

 which is circulated in these vessels consists of a liquid plasma in which 

 float colorless ameboid cells called corpuscles. In the plasma is dissolved 

 a red coloring substance known as hemoglobin which aids in the inspiration 

 and transportation of oxygen by combining with it, forming oxyhemo- 



Dorsa/ 

 6/ood 



Longitudinal 

 ■ muscles 



Typh/oso/e 



First toop^ o-f 

 nephr/'o/iurrt 



Thirc^ hop o-f 

 nephriof/urrt 



Second /oop ot- 

 nephn'i^iuni 



Nepffric/iopore- 



Position ot^ 

 ''n terseg/n^en ta/ 

 septum 



Cuticu/a 



Hypoo/e^-fTt/s 

 'rcu/ar musc/e 



^ /ntest/na/ 

 6/ooc^ vessel 



Ch/or-a - 



grogye 



ce//s 



Jfites^/ne 



testinai/ 

 9pithe//urT7 



f/ep t) rostome 



Vent fa/ 

 r?efVe cordis 



Y^- 



\ So&intest'>-'a/ 

 biooai vesse/ 



(j/arnt f/'/berS 

 Subneur-a/ h/ooaf vesse/ 



Fig. 145. — Diagrammatic cross section of an earthworm at about the middle of the 

 body. {From Marshall and Hurst, "Practical Zoologij," by the courtesy of G. P. Putnam's 

 Sons.) The nephrostome of the segment in front is shown for the sake of completeness. 



glohin. Much more oxygen can be contained and transported in com- 

 bination than if it were free. This combination is formed when oxygen 

 is taken into the body and is only temporary, being broken up again in 

 the tissues, thus hberating free oxygen for their use. Respiration takes 

 place through the whole surface of the body. 



273. Excretory System. — The excretory system consists of a number 

 of organs called nephridia (Fig. 147), a pair of which is present in every 

 metamere but the first three and last. Each of these possesses a cihated 

 funnel called a nephrostome located in the posterior part of the coelom 

 of one metamere and opening into a thin ciliated tube passing through 

 the septum into the coelom of the next metamere. Here the tube 

 becomes complexly looped and ultimately opens by a nephridiopore 

 between the two double rows of setae. Cilia on the nephrostome create 



