CHAETOPODA. 595 



podium and a ventral neuropodium, which are separated by a greater or 

 less interval and differ in size and form not only in different Polychaeta 

 but in different regions of the same individual. They may be absent in 

 certain regions in some Tttbicola, e. g. from the posterior part of the body 

 in Arenicola, &c. The cirri are in intimate relation with the parapodia, 

 and there is usually a dorsal or notopodial cirrus and a ventral or neuro- 

 podial. The latter is as a rule insignificant in size. The cirri of the 

 peristome, and following somites, and of the pygidial somite, are frequently 

 much lengthened and enlarged. The organs in question are solid processes 

 of the body-wall, probably tactile in function, with muscular walls, and con- 

 taining a nerve. They vary in form, and may be conical, filamentous, 

 lamellate, and are sometimes jointed. The Aphroditidae are distinguished 

 among Polychaeta by the presence of lamellate plates or elytra attached 

 dorsally to the notopodium. They contain a rich supply of nerves and 

 have been regarded as modified cirri, but the two structures sometimes 

 co-exist on the same somite 1 . The branchiae, like the cirri, are processes 

 of the body-wall, but they differ from them either by containing an ex- 

 tension of the coelome, or a single contractile vessel, or an afferent and an 

 efferent vessel which are connected by capillary loops or networks. They 

 are either filamentous, lamellate, pectinate, or branched in shape, and are 

 present either on the majority of the somites, or on certain somites only 

 in Tubicola, e.g. the anterior in Terebella or the median in Arenicola. 

 They are often entirely absent The prostomial tentacles of Terebellidae 

 must possess a respiratory function, inasmuch as they contain a cavity 

 which is an extension of the coelome. The cephalic gills of the Serpulidae 

 consist of a more or less distinct basal ridge, the two ridges right and left 

 being connected dorsally, from which spring a number of filaments, varying 

 from 4-50. These filaments usually support each two lateral rows of 

 ciliated processes. They may contain only an extension of the coelome, 

 with the exception of the two median dorsal filaments, which are traversed 

 by a single vessel, e. g. Haplobranchus, Manayunkia ; or all alike are traversed 

 by a similar single vessel which gives off a branch into each lateral process. 

 These vessels contract and dilate alternately. In the tribe Sabellinae both 

 ridges and filaments are supported by a cellular cartilage and the filaments 

 are connected by a membrane. In the Serpulinae, with few exceptions, 

 e. g. Protula, one, rarely both, of the median dorsal filaments is modified 

 into a stopper-like operculum, which closes the tube when the animal 



appear in the same position in the sexually mature Syllidae and Hesionidae. Pruvot concludes there- 

 fore that the simple or uniramose character of the pseudopodia in these worms is due to the atrophy 

 of the notopodium. 



It is a remarkable fact that the only setae found in Tomopteris are the single setae lodged in the 

 extremities of the cephalic appendages. See Greeff, Z. W. Z. xxxii. 1879, Taf. xv. Figs. 40, 41, 47. 

 The large pair of appendages are innervated from the cerebral ganglia (Pruvot). 



1 For the structure of the elytra in Polynoe, see Jourdan, Z. A. viii. 1885. 



Q q 3 



