264 



POLYCHAETA 



In a few cases, such as the Chlorhaemids and Sternaspis, and 

 to a slight degree in Arenicola, the " head " and even the anterior 

 part of the worm is capable of being withdrawn into the body. 



The Parapodia and Chaetae. The typical parts of a para- 

 podium have been described in the preceding chapter ; here it is 

 only necessary to refer to the series of diagrams (Figs. 136, 137) 

 representing the parapodia of the more common Polychaetes, and 

 to add a few remarks about them. 



In most Annelids the chaetae are in two bundles on each 



Fia. 136. Parapodia. A, Nephthys; E, Amphinome ; C, Glycera (the unlettered lobe 

 above g is the notopodial cirrus) ; D, Syllis ; E, Eunice ; F, Phyllodoce. , Noto- 

 podial cirrus ; b, notopodium ; c, neuropodium ; d, neuropodial cirrus ; g, special 

 gill ; n, aciculum (omitted in B) ; x, cirriform lip of chaetigerous sac. 



side, but there are certain families in which the dorsal bundle, 

 and even the notopodium itself, is absent, as in the Eunicidae, 

 Syllidae, and Phyllodocidae ; or the dorsal bundle may be absent 

 only in certain regions of the body, as in the hind-body of Tere- 

 bellids. In some Amphinomidae and Aphroditidae the noto- 

 podium is scarcely distinct as a separate lobe, being a slight 

 tubercle on the upper surface of the neuropodium ; but the noto- 

 podial chaetae are present, and indeed particularly well developed 

 in many cases. 



But whilst, in the Nereidiformia, the parapodia, whether con- 



