PHYLUM IV. 

 ANNELIDA.* (The Annelid Worms.) 



Elongated, segmented worms in which a distmct head, a coelom, and 

 a digestive tube are present, and the paired appendages, where present, 

 are not segmented. 



External Structure.— The segmentation, which is the most character- 

 istic feature of annelids, is approximately equivalent (Fig. 461, A). 

 This is the most pronounced in the Chcetopoda, in which it affects both the 

 inner and the outer structure, the segments or somites being separated 

 from one another by muscular partitions called dissepiments; in the 

 Hirudinea and the Myzostomida the internal structure is also completely 

 segmented but the rings which appear on the outer surface may not mark 

 the somites; in the EcJiiurida the larval worm alone is segmented. 



A head is more or less distinctly marked in most annelids and contams 

 the mouih, the brain, and also often tentacles, cirri and palps, which 

 may have a tactile and often a respiratory function; eyes are also often 

 present. The head is typically composed of two divisions (Fig. 459), the 

 prostomium and the peristomium or metastomium. The first of these 

 divisions is a distinct projection which forms the anterior end of the 

 animal; it lies in front of the mouth (Fig. 450, A) and contains the brain 

 and the eyes, tentacles, and palps, if these are present. The second con- 

 tains the mouth, which is ventral in position, and. often cirri, and is con- 

 tinuous with the segmented trunk, in many annelids appearing as a 

 part of it. 



Paired, segmental appendages, which in the annelids are muscular 

 projections of the body wall and are called parapodia, are present in the 

 PolycTiceta and the Myzostomida, and all annelids except the Hirudinea, 

 most Archiannelida, and the Discodrilidae , possess paired, segmental groups 

 of bristles, which are called setae and assist in locomotion. The parapodia 

 are locomotory, respiratory, and tactile in function. The Hirudinea and 

 a few other groups possess suckers, which assist in locomotion. 



The outer surface of the body of annelids is a cutieula and is usually 

 not ciliated in the adult worm. 



* See "Invertebrate Animals of Vineyard Sound," by A. E. Verrill, Rep. TJ. S. 

 Com. Fish, for 1871-72. "Preliminary Account of the Marine Annelids of the Pacific 

 Coast" etc., by H. P. Johnson. Proc. Cal. Acad. Sci. (3), Vol. 1, 1897. "A Biological 

 Survey of the Waters of Woods Hole and Vicinity," by F. B. Sumner, et al., Bull. Bur. 

 Fish., Vol. 31, 1913. 



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