82 



ANIMAL BIOLOGY 



tory system of blood vessels that propels and distributes blood to 

 all parts of the body : an efficient transport system for carrying ab- 

 sorbed food to every organ, waste products to the excretory organs, 

 and carbon dioxide to, and oxygen from the respiratory surface. 

 And when we add the advances in digestive, nervous, and excretory 

 systems, the Segmented Worms approach more nearly the popular 



Tentacles 



Prostomium bearing 

 four eyes 



Parapodia 



MouUx 



Posterior sucker 



A B 



Fig. 45. — A, Sandworra, Nereis virens; B, Medicinal Leech, Hirudo medicinalis. 



concept of an animal than the Invertebrates we have heretofore 

 met. 



And withal, in variety and numbers the phylum is second to 

 few. Some idea of the importance of Earthworms may be gathered 

 by Darwin's estimate that in many soils they annually bring to 

 the surface some eighteen tons of earth per acre. Not unimpor- 

 tant transformations of the surface soil is' thus effected: indeed 

 Earthworms, unseen and unheard, perform a vitally important 

 part in loosening and aerating — in plowing — the soil. 



F. Rotifers, Bryozoans, and Brachiopods 



At this point in our survey of the Invertebrates it is opportune 

 to take merely a passing glance at several aberrant groups whose 

 position in the series is decidedly uncertain, and which contribute 

 little of theoretical or practical importance. 



