180 THE VITAL FUNCTIONS. 



In proportion as the changes of form which the insect un- 

 dergoes are less considerable, the evidences of a circulation 

 become more distinct. Such is the case in many of the Ap- 

 terous Insects, composing the family of Myriapoda: in the 

 .Scolopendra morsitans, (Linn.,) for instance, Duges ob- 

 served the dorsal vessel dividing into three large branches. 



Most of the tribes belonging to the class of Arachnida 

 have, likewise, a dorsal vessel, very analogous in its struc- 

 ture and situation to that of insects; and, as none of them 

 undergo any metamorphosis, their vascular system admits 

 of being considerably developed, and becomes a permanent 

 part of the organization. Fig. 346 shows the dorsal vessel 

 of the iflranea domestica, or house spi- 

 der, with some of the arterial trunks 

 arising^from it, lying embedded 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 co- 

 vering 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 larva? of insects. 

 A complete vascular circulation is established in all the 

 animals which compose the class of Annelida; the vessels 

 being continuous throughout, and having sufficient power to 

 propel the blood through the whole of its circuit. Great va- 

 riety exists in the arrangement and distribution of these 

 vessels, depending on the form of the animal, the compli- 

 cation of its functions, and the extent of its powers. The 

 first rudiment of a distinct system of circulating vessels, in- 

 dependent of the ramified tubes proceeding from the intes- 



