CIRCULATION IN THE ARACHNIDA. 



249 



tica, or house spider, with some of the arte- 

 rial trunks arising from it, lying 

 imbedded in a thick mass of 

 substance, having a similar oily 

 character to that which is con- 

 tained in large quantities in 

 the principal cavities of insects- 

 It is, in general, difficult to ob- 

 tain 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, by 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. 



A complete vascular circulation is established 

 in all the animals which compose the class of 

 Annelida ; the vessels being continuous through- 

 out, and having sufficient power to propel the 

 blood through the whole of its circuit. Great 

 variety exists in the arrangement and distribu- 

 tion of these vessels, depending on the form of 

 the animal, the complication of its functions, 

 and the extent of its powers. The first rudi- 

 ment of a distinct system of circulating vessels, 

 independent of the ramified tubes proceeding 



