184 THE ANNELIDA POLYCHAETA. 



species belonging in the Hesionidae as well as others in the Syllidae. Various 

 species have been referred to one family by some authors, to the other by others. 

 The more specialized genera are, of course, readily placed. 



The hesionids are only rarely taken at more than moderate depths, where 

 they occur commonly on beds of broken shell and coral, in the fissures of rocks, 

 and in tangles of seaweeds, while a fondness for mud has been noted for various 

 species {e.g., Leocrates atlantica). Some forms are commensal, such as Leocra- 

 tides filamentosus Ehlers living in a siliceous sponge {AphrocalUstes bocagei) 

 (Ehlers, Deutsch. tiefsee exped. Valdivia, 16, 1908, p. 64), Dalhousiella carpin- 

 tieri Mcintosh also living in siliceous sponges (Cf. Fauvel, Result. Camp. sci. 

 Prince Monaco, 1914, 46, p. 124), and Oxydromus flexuosus Delle Chiaji living 

 in the ambulacra of Astropecten' aurantiacus and other species of the same genus 

 {Cf. St. Joseph, Ann. sci. nat., 1888, ser. 7, 5, p. 327) . 



Some hesionids, at least, swim readily and actively. The diverticula aris- 

 ing behind the oesophagus, as previously mentioned, are cormnonly much dis- 

 tended with gas and so may have an hydrostatic function in addition to the 

 respiratory function that compensates for lack of branchiae in these forms (see 

 Eisig, Mitth. Zool. stat. Neapel, 1881, 2, p. 255). They often do not sink when 

 transferred to fixing or other fluid until bubbles of gas have escaped from the 

 alimentary track. They engulf water as well as air, and are actively carnivorous, 

 preying upon their own kind as well as upon other annelids. Many are highly 

 active and irritable, darting forward or backwards with equal readiness when 

 touched, and if much irritated throwing off appendages or breaking into pieces, 

 such autotomy being a pronounced characteristic in such forms as have been 

 observed in life. The result is that it is difficult to secure specimens entire. 



The brilliant colors shown by many species of this family have no obvious 

 connection with their habits. Thus the gaudily colored Castalia punctata occurs 

 in deep water and clings persistently to the crevices and under surfaces of rocks, 

 w^here opportunity for display is meagre. Oftentimes a specially striking color 

 or marking is apparent only at the period when sex-products are carried. Thus 

 the parapodia in the species mentioned become beautifully pink in September 

 from the masses of ova extending into them from their bases. Castalia fusca, 

 ordinarily yellowish brown or fawn colored, when carrying ova in May assumes 

 a mauve or purpUsh pink hue which disappears when the ova are discharged 

 {Cf. Mcintosh, British annehds, 1908, 2, pt. 1, p. 124, 129). In some the colors 

 change readily when the sea-water is diluted or otherwise changed; and reversi- 

 bility is probable. As with the Syllidae numerous genera have been proposed 



