vi RESPIRATION AND EXCRETION 339 



trunk 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 plentifully 

 supplied with blood. 



The red colour of the blood is due to hemoglobin (p. 

 107), w r hich is not, as in the frog, contained in red blood- 

 corpuscles, but is dissolved in the plasma, in which, how- 

 ever, minute colourless corpuscles can be recognised. The 

 function of haemoglobin in the process of respiration has al- 

 ready been described (p. 144) ; but in the earthworm, as in 

 many other lower animals, there are no specialised res- 

 piratory 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 Amceba 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 

 Paramcecium or Vorticella, and are more nearly comparable 

 with those of the frog (p. 146). 



z 2 



