Chap. 28 ANNELIDS PIONEERS IN SEGMENTATION 561 



Respiration. Earthworms can breathe in air or water. Their wet skin func- 

 tions essentially like a lung or a gill in spite of its cover of cuticle. It is well 

 supplied with blood capillaries and under sufficient pressure oxygen passes into 

 the blood and combines with the hemoglobin; in the outer skin cells it prob- 

 ably combines directly with the protoplasm as in the ameba. Although no 

 exact measurements are available, it appears that earthworms can make use 

 of oxygen in air or water with almost equal readiness. Experiments have shown 

 that the oxygen-loading capacity of the earthworm's hemoglobin is low and 

 inefficient as compared with the hemoglobin of higher animals. 



Excretion. The nephridia (kidneys) of annelid worms are tubes associated 

 with blood vessels and with the coelomic fluid (Fig. 28.9). Each one is a coiled 

 tube with a ciliated funnel opening into the coelom, and a relatively long tube 

 looped back and forth upon itself and ending in an enlarged bladderlike part 



Dorsal vessel 

 Chlorogogen cells \ 



Endoderm 

 Muscle \ 



Peritoneum 



Cuticle 



I Ectoderm 

 I 



Circular muscle 



Longitudinal muscle 



Peritoneum 

 I 

 I 



Typhi 



/ 

 Seta 



Nephridiopore 



y Ventral vessel 

 taterol vessel 



\ \ 



^ Ventral nerve cord 



Subneural vessel 



\ ''Coelom 

 Enteron 



Fig. 28.9. Diagram of a cross section of an earthworm showing the intestine, 

 one pair of nephridia, the chitinous setae which are aides in locomotion and the 

 excretory chlorogogen (chloragog) cells. The inbent fold (typhlosole) extending 

 nearly the whole length of the intestine is a means of increasing the area of 

 absorption of digested food. None of the smaller blood vessels are shown; nets of 

 them cover the coils of the nephridia. (Courtesy, Mavor: General Biology, ed. 3. 

 New York, The Macmillan Co., 1947.) 



