266 POLYCHAETA 



become flattened and extended antero-posteriorly, and unite with 

 one another to form the " thoracic membrane." l In Phyllo- 

 docidae the cirri are foliaceous and natatory, and they contain 

 a great quantity of glands of a peculiar character. The Aphro- 

 ditidae are distinguished from other Annelids by the possession 

 of " elytra " or dorsal scales, which appear to be the dorso- 

 ventrally flattened cirri, retaining their sensory nature, but 

 adding to this function several others. 2 



The chaetae or bristles are mainly used in locomotion, but it 

 is not unreasonable to believe that some of the stronger, serrated 

 kinds may be used as weapons of offence and defence ; certainly 

 the Polyiioids, bristling as they do with stiff chaetae along each 

 side, must be rather unpleasant to their smaller enemies. 



The various bristles may be placed in three chief groups, viz. 

 (1) simple; (2) jointed; (3) uncini (see Fig. 138). 



(1) The simple chaetae may be smooth and hair-shaped, i.e. 

 " capillary," such as are present in nearly all families : or they 

 may be forked (Amphinomidae), comb-shaped (Eunice}, notched 

 or serrated, or provided with a series of frills at right angles to 

 their length, as in Aphroditidae ; or fringed along one or both 

 sides with a membranous expansion, as in Terebellids and 

 Sabellids. The simple chaetae may also be short and spine-like, 

 as in the ventral bundles of Arenicola ; or they may be slightly 

 curved at the end and notched, forming what are generally 

 termed " crotchets," such as are common amongst Oligochaeta. 

 These " crotchets " may be simple, or have numerous denticula- 

 tions at the end (Maldanidae), or be provided with a membranous 

 hood (Spioniformia, Capitelliformia). In ffermione peculiar 

 sheathed, spear-like bristles occur (Fig. 138, N). 



(2) Jointed chaetae have already been described (p. 246); they 

 are confined to the sub-Order Nereidiformia, and occur only in 

 certain families. 



(3) The uncini are very short chaetae, which are simply 

 embedded in the skin, and do not extend beyond the body-wall 

 into the body-cavity. An uncinus is a sharply curved hook, which 

 may have more or less numerous secondary teeth on it. They 

 are characteristic of the Sabelliformia and the Terebelliformia. 



The chaetae appear as solid, usually fibrillated structures, of 

 a yellow or golden tint, transparent and refringeut. Chemically 

 1 Meyer, loc. cit. " Haswell, P. Linn. Soc. tf.S. Wales, vii. 1883, p. 251. 



