PHYLUM ANNELIDA 



The phylum Annelida comprises the so-called "true worms'' 

 in contrast to the flat worms, liotii phyla are important in 

 tracing advances in organization. 



In the flat-worms bilateral symmetry and progression with 

 the same end foremost are associated with noticeable though 

 moderate regional diff"crentiation, as for example, an anterior 

 end which may be regarded as an incipient head. In the annelids 

 the anterior end has become further differentiated. Aside from 

 this advance in their perfection the annelids are organized upon 

 the structural plan of metamerism accompanied bj' the presence 

 of a true body cavity or coelom which is not to be regarded as 

 the least of important annelid innovations. 



Class Chaetopoda (Nereis, a sandwor7n). — Nereis may be 

 taken as a typical annelid. It lives in burrows in the coarse 

 sand and mud of the seashore at tide level. 



External Structure. — Study an animal in a wax-bottomed 

 dissecting pan. Make observations upon the surface structure 

 with a view to determining the kind of symmetry. The most 

 striking structural character is the ringed appearance of the 

 body. Each of these rings is a unit in the organization plan of 

 the body and is called a metamere (or somite or segment). Each 

 metamere bears a pair of very thin lateral prolongations of the 

 body wall, known as parapodia, whose double function will be 

 considered later. 



Head. — Two segments in the anterior region are so modified 

 as to be able to test the nature of the surroundings as the body 

 advances. Other functions in these segments are lost to a great 

 extent. The first somite, or prostomium (meaning "in front of 

 the mouth"), is a dorsal rectangular projection which does not 

 extend to the ventral side. The second, or peristomium, is a 

 complete ringlike segment bearing the mouth. 



The sense organs of the head have reached a relatively high 

 state of perfection. Projecting from the middle of the cephalic 

 edge of the prostomium there are two short prostomial tentacles. 

 On either side of these is a thick, jointed palp whose service is 



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