CHJETOPODA. 



867 



metamorphosis. The Annelida comprise terrestrial and aquatic 

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

 them (flirudinea) are occasionally parasitic. 



In the group of the Annelida three principal divisions may be 

 distinguished, the Chcetopoda, the unsegmented 

 Gephyrea, 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 Oligochreta, from which they 

 may be derived. 



/Sub-class 1. CH^TOPODA.* 

 Free living Annelida, with paired tufts of 

 setce on the segments, frequently with distinct 

 head, also with tentacles, cirri, and branchice. 



The Chsetopoda 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 setae are very frequently present on the segments ; their prin- 



cipal function is that of 

 locomotion, but their va- 

 rious appendages, the 

 branchice and cirri, also 

 discharge tactile and respi- 

 ratory functions (fig. 297). 



FIG. 296. Grulea fun- 

 fera (after Quatre- 

 fages). Ph. pharynx 

 D, alimentary canal; 

 C, cirri; JF, tentacles. 



Ac 



* Besides the older works 

 of Savigny, Audouin et Milne 

 Edwards, and Quatrefages, 

 compare E. Grube, " Die F ami- 

 lien der Anneliden," Archiv 

 fur Naturr/eseJt, 1850 and 1851. 

 E. Claparede, " Eecherches 

 anatomique sur les Ann glides, 

 etc.," Geneve, 1861. E. Cla- 

 parede, " Les Annelides che'to- 

 podes clu golfe de Naples," 

 Geneve et Bale, 1868, also Sup- 

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

 Geneve, 1873. Fr. Leydig, 1. c.. also " Tafeln zur vergl. Anatomic/' 1864. 



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

 pcdium with bundles of setaB of Nerds (after 

 Quatrefages). Ac, Aciculum ; Re, dorsal cirrus j 

 Be, ventral cirrus. 



