330 THE EARTHWORM chai-. 



blood from the enteric canal is returned by a series of vessels 

 into the dorsal trunk. The vessels of the excretory organs, to 

 be described presently, arise from the ventral trunk, and the 

 blood from these organs is returned into 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 hmmoglobin (p. 

 107), which 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 occur- 

 rence, 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, we found (p. 255) 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 the ciliate Infusoria. 

 But the reader will have noticed that Hydra and its 

 allies have no specialised excretory organ, waste pro- 

 ducts 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, 



