APPENDAGES OF VERMES. 



133 



world ou tlie organism. They are especially developed in the Chseto- 

 poda, the cephahc region of which is provided with contractile pro- 

 cesses, either at the sides or in the middle line (Fig. 55, t t'). These 

 processes are simple, or are further differentiated by segmentation, 

 or even distinguished by the possession of secondary processes. By 

 adaptation to the most varied conditions of life they are converted 

 into very various structures, and serve for all kinds of functions. 



In the tubicolous Chastopoda, where the cephalic region is that por- 

 tion of the body which comes into closest relation with the surround- 

 ing medium, the tentacles are converted into an important apparatus. 

 They form tufts of contractile filaments on the cephalic lobes, where 

 they are arranged 

 in one or more 

 rows (Terebella 

 [cf . infra. Fig. 79, 

 t], Hermella); or 

 they may be con- 

 verted into strong 

 plume-like struc- 

 tures (branchial 

 tentacles) by the 

 development of an 

 internal support 

 (cartilage), and be 

 laeset with secon- 

 dary branches ; 

 these, in addition 

 to their respira- 

 tory function, may 

 also aid in the 

 movement of the 

 whole apparatus 

 when seizing food 

 (Serpulacese). In 

 some, these bran- 

 chial tentacles are 



arranged in two groups, and resemble an open fan. In Siphonos- 

 toma they form short simple filaments, with two longer delicate 

 feelers. In others the base of the two halves of the tuft, which are 

 separated on the dorsal line, is drawn out into a spirally-coiled 

 ridge, on which the separate filaments are arranged (Sabellidfe). 

 When optic organs are formed on the separate filaments of the 

 branchial tufts, the tentacles acquire new and important relations 

 (Branchiomma) . 



Some of the branchial filaments undergo other kinds of changes. 

 In some Sabellidte one or two of the primitively similar branchial 

 tentacles (Protula) lose their respiratory function ; in others they are 

 converted into club-shaped organs, one of which is largely developed, 

 and serves as an operculum to close the tube in which the animal 



Fig. 55. Head of Nereis Dumerilii. aa Tentacles. 



t t' f t'" i'^ i^ Feelers, p Parapodia. p/i Pharynx. 



m Jaws, i (Esophagus, gl Glands (after Claparfede). 



