CHAETOPODA 



231 



number of families into which the Polychaeta are divided, in which 

 a rough oecological grouping is adopted. A summary of the variation* 

 in segmental organs and reproductive habits follows at the end. 



fEunicidae. Eunice, Leodice 



The errant Polychaeta with unmodified 

 head and armed eversible pharynx; 

 fitted for an active life but often living 

 in tubes ; very often greatly modified 

 in structure and physiology at the 

 sexual season. 



The true tubicolous Polychaeta, much 

 modified for the collection of micros- 

 copic food ; anterior part of gut not 

 eversible and jaws absent ; inhabiting 

 tubes which they rarely or never 

 leave. 



The burrowing Polychaeta with re- 

 duced head; with proboscis. 



(the Palolo worm). 

 Nereidae. Nereis, 

 Syllidae. Syllis, Myri- 



anida. 

 Phyllodocidae. Eulaliay 



Aster ope. 

 Polynoidae. Aphrodite^ 



Lepidonotus , Panthalis . 



'Chaetopteridae. Chaeto- 



pterus. 

 Terebellidae. Terehella, 



Amphitrite. 

 Serpulidae. Pomatoceros y 



Filigrana. 

 Sabellidae. Sabella^Spiro- 



gr aphis. 



IArenicolidae. Arenicola 

 without jaws. 

 Glyceridae. Glycera with 

 jaws. 



The errant Polychaeta 



The external structure is known to the elementary student through 

 the type Nereis (Fig. 173). The prostomium bears two kinds of 

 filiform, tactile appendages, the tentacles which are dorsal and the 

 palps which are ventral ; there are also one or two pairs of eyes upon 

 it. The anterior part of the gut [pharynx) is eversible and serves for 

 grasping food ; its lining may be chitinized in places to form the jaws 

 and paragnaths oi Nereis or teeth as in Syllis. These are not necessarily 

 the sign of a carnivorous habit but may be used for cutting up pieces 

 of seaweed or boring in sponges. 



The ordinary trunk segment has a double parapodium consisting 

 of a dorsal notopodium and a ventral neuropodium, usually with rather 

 different types of chaetae. A dorsal cirrus and a ventral cirrus are 

 nearly always present ; they are filiform structures but may be modi- 

 fied to form pectinate gills (Eunice) or plate-like elytra (Polynoidae). 

 From the conical noto- and neuropodia spring a bundle of chaetae ; 

 the chaetal sacs project into the coelom and each bundle is supported 

 by an enlarged and wholly internal chaeta — the aciculum^ which also 



