FLABELLIGERIDAE. 395 



The prostomium is small and often retracted into the peristomium. It 

 bears a pair of palpi, which are in general grooved. Above, the tentacles or 

 branchiae are short and filiform, and more or less numerous, always four 

 or more, these extending forward about the prostomium and palpi. Eyes nor- 

 mally four, though easily overlooked because of the frequent retraction of the 

 prostomium. 



The peristomium is generally short, but it may be extended into a short 

 and retractile tube. It is setigerous. 



The parapodia are biramous, with setigerous papillae usually absent or 

 obsolete, but sometimes distinct. No true cirri. 



The setae are all characteristically strongly cross-striate, or annulatcd. 

 They are of two principal types, slender capillary ones, and shorter, stout acicular 

 ones, or crochets. The capillary setae are always simple; the crochets either 

 simple or composite. The bristles of the first few somites in most cases are 

 very long and directed forwards so as to form the so-called cage about the ante- 

 rior end; these setae often have a special structiu"e and a characteristic iridescence. 



The nephridia are two tubes ending each in a cul-de-sac near the sides of 

 the stomach and opening separately or by a common pore anteriorly near the 

 mouth (Haswell, Proc. Linn. soc. N. S. W., 1892, ser. 2, 6, p. 349). 



The blood-fluid when seen in thin layers, or as it appears in the animals in 

 life, is green, the color being due to a substance allied to haemoglobin and termed 

 chlorocruorin. In quantity it is dark red. In alcohol, the blood commonly 

 appears reddish.^ 



The alimentary tract presents anteriorly a narrow oesophagus with ciliated 

 lining, behind this a wide, thin-walled stomach, with an anteriorly projecting 

 coecum, followed by a narrow, thick-walled intestine, in which there is normally 

 a more or less strongly marked sigmoidal flexure. 



The flabelligerids hve largely in muddy ground, or muddy sand, and in 

 slime, at various though mostly small or moderate depths. They may occur 

 between tide-marks on the one hand, while they may descend to great depths 

 on the other, the remarkable Buskiella of the Challenger expedition occurring 

 at a depth of 2,500 fms. (Mcintosh, Challenger Annehda, 1885, p. 372). The 

 Uttoral forms are dominantly inhabitants of the colder regions. Thus, as 

 Ehlers points out (Festsch. K. gesellsch. Gottingen, 1901, p. 179), of the nine 

 species mentioned by Levinsen (Vidensk. meddel. foren. Kjoben., 1883, p 301) 

 in his review of the flabelligerids of the North Sea, six are Arctic. Carus gives 



1 Cf. Lankester, Proc. Royal Soc, 1873, 21, p. 2; Mcintosh, British annehds, 1915, 3, pt. 1, p. 85. 



