CUMTOPODA. 



867 



metamorphosis. The Annelida comprise terrestrial and aquatic 

 animals, and they eat, for the most part, animal food. Many of 

 them (Hirudinea) are occasionally parasitic. 



In the group of the Annelida three principal divisions may be 

 distinguished, the Chcetopoda, the unsegmented 

 Gepliyrea, and the Hirudinea which are adapted 

 for parasitism. The Hirudinea are not in any 

 .degree to be regarded as Annelida of a lower 

 grade of organization, but they rather present, 

 at least in the case of some organs, as alimen- 

 tary canal, circulatory and generative organs, 

 a more complicated structure, and agree most 

 closely with the Oligochteta, from which they 

 may be derived. 



Sub-class 1. CH.ETOPODA.* 

 Free living Annelida, ivith paired tufts of 

 setcc, on the segments, frequently with distinct 

 head, also with tentacles, cirri, and branchice. 



The Chtetopoda are divided externally into 

 segments, which correspond with the metameres 

 of the internal organs, and are, with the excep- 

 tion of the anterior region, which is distinguished 

 as the head, usually tolerably alike (fig. 296).- Parapodia provided 

 with setaj are very frequently present on the segments ; their prin- 

 cipal function is that of 

 .DP _ locomotion, but their va- 



rious appendages, the 

 branckice and cirri, also 

 discharge tactile and respi- 

 ratory functions (fig. 297). 



* Besides the older works 

 of Savigny, Audouin et Milne 

 Edwards, and Quatrcfapes, 

 compare E. Grube, " Die Fami- 

 lien dcr Anneliden," Arrliir 

 fur Naturgesclt, 1850 and 1851. 

 E. Claparedc, " llecherches 

 anatomiquc sur les Annelides, 

 etc.," Geneve, 1*01. E. Cla- 

 parede, ' Les Annelides cheto- 

 podes du golfe de Naples," 

 Geneve et Bale, 1868, also Sup- 

 plement, 1870, and " Eecherches sur la structure des Annelides sedentaires," 

 Geneve, 1873. Fr. Lcydig, 1. c.. also " Tafeln zur vergl. Anatomic," 1SG1. 



FIG. 296. Grulea 

 fera (after Quatre- 

 fages). Ph. pharynx 

 D, alimentary canal ; 

 C, cirri; F, tentacles. 



Ae 



FIG. 297. Dorsal (DP) and ventral (VP) Para- 

 pi dium with bundles of setae of Nerds (after 

 Quatrefages). Ac, Aciculum ; B.C, dorsal cirrus ; 

 Be, ventra' cirrus. 



