vr RESPIRATION AND EXCRETION 339 



and the commissural vessels in the body-wall which 

 connect the dorsal and subneural trunks. By means of 

 branches of these parietal vessels the body-wall is plenti- 

 fully supplied with blood. 



The red colour of the blood is due to hemoglobin 

 (p. 107), which is not, as in the frog, contained in red 

 blood-corpuscles, but is dissolved in the plasma, in 

 which, however, minute colourless corpuscles can be 

 recognised. The function of haemoglobin in the process 

 of respiration has already been described (p. 144) ; but 

 in the earthworm, as in many other lower animals, there 

 are no specialised respiratory organs (lungs or gills), 

 the necessary exchange of gases being performed by 

 the entire surface of the body, the minute branches of 

 the blood-vessels in the body-wall being only separated 

 from the air by the single -layer of epidermic cells and 

 even penetrating amongst the latter in the region of 

 the clitellum : this is an exceptional occurrence, for, as 

 we have seen, capillaries do not, as a general rule, extend 

 amongst epithelial cells (compare, e.g., Figs. 38-40). 



In discussing in a previous chapter the differences 

 between plants and animals (p. 255), we found that in 

 the unicellular organisms previously studied the presence 

 of an excretory organ in the form of a contractile vacuole 

 was a characteristic feature of such undoubted animals 

 as Amoeba and many other Protozoa. But the reader 

 will have noticed that Hydra and its allies have no 

 specialised excretory organ, waste-products being 

 apparently discharged from any part of the surface. 

 In the earthworm we meet once more with an animal 

 in which excretory organs are present, although, in 

 correspondence with the complexity of the animal itself, 

 they are very different from the simple contractile 

 vacuoles of Paramoecium or Vorticella, and are more 

 nearly comparable with those of the frog (p. 146). 



Z 2 



