CIRCULATION IN THE ARACHNIDA. 229 



which, as they undergo no metamorphosis, consti- 

 tutes a permanent part of their organization. Fig. 

 346 shows the dorsal vessel of the Aranea domestica, 

 or house spider, with some of the arterial trunks 

 346 .™»,_,«^ arising from it, lying imhedded in a 



thick mass of substance, having a 

 similar oily character to that which 

 is contained in large quantities in 

 the principal cavities of insects. 

 It is, in general, difficult to obtain 

 a view of the circulation in the 

 living spider, on account of the 

 thick covering of hair which is 

 spread over the body and the limbs ; but if a species, 

 which has no hair, be selected for examination, we 

 can see very distinctly, through the microscope, the 

 motion of the blood in the vessels, bv means of the 

 globules it contains, both in the legs and in other 

 parts, where it presents appearances very similar to 

 those already described in the limbs of the larvae of 

 insects. 



The simplest form of a complete vascular cir- 

 culation is presented in the Annelida. Their vessels 

 are so disposed that the blood, receiving the in- 

 fluence of the air principally through the medium 

 of the integuments, or of respiratory organs imme- 

 diately under the surface of the body, pursues its 

 course along the back and sides of the body with- 

 out interruption : and consequently the distinction 

 between arteries and veins is altogether lost among 

 the minute branches of that network which com- 

 poses the principal part of the vascular system. 

 The carbonised and the oxygenised portions of the 



