PHYLUM COELHELMINTHES (ANNELIDA) 



153 



Various systems of subdividing the Oligochaeta have been 

 proposed, most of which have as a basis either the habits or 

 structural characters correlated with aquatic or terrestrial habits. 



Members of the order Microdrili are small oligochaetes of 

 relatively few segments and usually aquatic in habits. Eyespots 

 are frequently present. 



In the order Megadrili are found the larger oligochaetes, which 

 are commonly known as earthworms. The bodies of these con- 



FiG. 78.- 



-Flashlight photograph of earthworms in copulation. 

 by Alvin R. Cahn), 



{Original photo 



tain numerous segments (Moniligaster, Perichaeta, Microscolex, 

 Diplocardia, Helodrilus, Lumbricus). 



Class Hirudinea 



The Hirudinea, or leeches, are annehds with a fixed number of 

 somites (generally 34), but superficially each somite is subdivided 

 by constrictions (Fig. 79) into a number of annulations, so the 

 number of external rings does not correspond to the number of 

 internal divisions of the coelom. The coelom is very greatly 

 reduced but the small pouches furnish the basis for the determina- 

 tion of the number of somites. With the exception of one genus 

 (Acanthobdella) leeches have no setae. The posterior extremity 

 bears a sucker on its ventral surface and usually a second sucker 

 is developed around the mouth. Both of these are organs of 

 attachment, and in addition the oral sucker aids in the ingestion 

 of food. In addition to a graceful undulating swimming, free 

 in the water, leeches may also progress by fixing the anterior 



