1 66 



EARTHWORMS. PHYLUM ANNELIDA 



stituents of this urine are urea and ammonia. These are probably 

 derived from both blood and coelomic fluid, and there appears to 

 be tilt rat ion, reabsorption and secretion, much as in the vertebrate 

 kidney (p. 379)- 



BLOOD VESSELS 



Earthworms have no special respiratory organs, but an inter- 

 change of gases between the air and the blood takes place in the 

 skin, which is richly supplied with vessels. 



hi. 



int. 



oes. 



b.w. - - jmiiimimmiiiiimiDiiiiiii, 



Fig. 115. 



e/^i>.it:v pMr.v. a//n.v. 



-A diagram of the blood- vascular system of the earthworm. 



aff.i.v., Afiferent vessels of the intestine ; aff.n.v., afferent vessels of the nephridia: b.w., body-wall; 

 d.b.v., dorsal blood vessel; .s.v., dorso-suDneural vessel ; ejj.b.w.v., efierent vessel from body-wall ; 

 eff.t.v., efierent vessel from intestinal wall ; hi., pseudo-hearts ; int., intestine ; oes., oesophagus ; 

 par.v., parietal vessel ; s.i.v., subintestinad vessel ; s.n.v., subneural vessel ; v.n.c, ventral nerve cord. 



A simpler form of this diagram will be found below. 



The blood of an earthworm is red owing to the presence in it 

 of a substance generally called haemoglobin, although it is different 

 in composition from the pigment of the same name in vertebrates. 

 This haemoglobin is in solution, not in corpuscles. Colourless 



cLb.y. ht. 



\SUp.pil.^ 

 ph. 



dJ.v. 



S.V»7 



xrui 



K7UC~ 



i-^ 



Fig. 116. — A diagram of the principal blood vessels of the earthworm. 



d.b.v., dorsal blood vessel; d.s.v., dorso-subneural vessel; ht., one of the 'hearts'; int., intestine; 

 m., mouth ; as., oesophagus ; ph., pharynx ; sup.ph.g., suprapharyngeal ganglion ; s.i.v., subintestinal 

 vessel ; s.n.v., subneural vessel ; v.n.c, ventral nerve cord. 



corpuscles are also present. The blood- vascular system is very 

 complicated. Its main outlines are as follows (Figs. 115-117). A 

 large dorsal vessel runs the whole length of the body from the 

 hinder end to the pharynx. It is contractile and its walls contain 

 muscle fibres, and in it the blood is driven forwards. It receives 

 blood by many small vessels from the intestine and by two larger 



