238 



ZOOLOGY 



in utilizing debris and the more minute organisms as food and thus becoming 

 a link in the saving of these to serve as food for the higher organisms cannot be 

 overestimated. (Pigs. 107 and 108.) 



Subclass II. OUgochcsta {with few bristles). — These are Ch^topoda with no 

 parapodia and comparatively few setas which usually occur in two or four clusters 

 in each segment. They are mostly fresh water or terrestrial in habit. Fleshy 

 outgrowths, such as gills, are almost universally absent. The sexes are united in 

 one individual and the accessory reproductive organs are very complicated. Ova- 

 ries and testes limited to a small number of anterior segments; development 

 direct. The anterior end not so highly specialized as in the Polychseta. 



Pig. 108. 



Fig. 108. Cirratulus grandis, from Verrill. 



Questions on Figs. 107 and 108. — Are these Chaetopods? What are your 

 evidences? What is the nature and function of the numerous outgrowths {bran- 

 chial cirri)} ,In what respects are they differently arranged in the two types? 

 Are these Oligochseta or Polychaeta? Your reasons? 



The earthworms, of which there are numerous species, are the best known types 

 of this subclass. The genera and species are distinguished chiefly by the position 

 of the sexual organs. The aquatic Oligochasta, which are much smaller, are found 

 in practically all ponds and ditches where organic matter is decaying. The more 

 common genera are Dero (Fig. loi), a beautiful, almost transparent worm which 

 often forms a temporary tube for itself of particles cemented by its own secretion, 

 and Tubifex, a longer worm which burrows in the mud at the bottom of streams; 

 a portion of the body protrudes from the inud and waves gently back and forth 

 in the water. They may occur so thickly that thousands may be seen in the space 

 of a few feet. When their home is jarred they speedily withdraw from sight. A 



