268 



POLYCHAETA 



of the notopodium of the eighth and ninth segments are 

 specially modified ; they are analogous to the copulatory chaetae 

 of Oligochaeta. In Aphrodite, in addition to the ordinary loco- 

 motor chaetae, there are brilliant, iridescent bristles and peculiar 

 felting threads arising from the indistinct notopodium ; these 



latter, however, are 



2 f' 



3 ^^ not true " chaetae, 

 but are separate 

 chitinous filaments 

 similar to the con- 

 stituent fibres of an 

 ordinary chaeta. 1 



While the chaetae 

 in the Nereidiformia 

 and others are 



FIG. 139. Aphrodite. Foot, x 2. a, Elytron ; b, noto- grouped in bundles, 

 podium ; c, neuropodium ; d, neuropodial cirrus ; n, aci- ~ 

 culum ; 1, iridescent bristles ; 2, stiff chaetae ; 3, felt, those OI many Other 



families are in ver- 

 tical, transverse rows, as in Maldanidae and in Arenicola. The 

 uncini are always embedded in such rows, usually slightly raised 

 from the general level of the body surface, each being termed 

 a " torus uncinigerus." These tori are usually limited to the 

 sides of the body, but in Myxicola and in Notomastus they 

 encroach upon the dorsal surface, and in Chaetozone, also upon 

 the ventral, so as nearly to encircle the body, recalling the 

 " perichaetous " condition of some earth-worms. 



Gills. We have already seen that several different organs, 

 e.g. the palps in Sabelliformia, the prostomial tentacles of 

 Chlorhaemidae, and the notopodial cirri of sundry other Poly- 

 chaetes, may take on a respiratory function. There are, however, 

 certain "gills" developed either on the parapodium itself or 

 elsewhere on the body which it is difficult to homologise. Such 

 are the retractile gills on the parapodia of the Glyceridae (Fig. 

 136, C) ; those of Dasybranchus, near the abdominal neuropodia ; 

 those of Mastobrarwhus, near the notopodia. Nephthys has a 

 sickle-shaped gill on the under - surface of the notopodium. The 

 long gill filaments at the posterior end of Sternaspis, again, are 

 only doubtfully interpreted as the dorsal cirri of some of the 

 posterior segments. 



1 Eisig, " Die Capitelliden," Fauna u. Flora G. v. Ncapel, Monogr. xvi. 1887, p. 331. 



