264 THE INVERTEBRATA 



The Chaetopoda are, in this work, divided into the following 

 orders: (i) Polychaeta, (ii) Oligochaeta. To the latter, however, the 

 Hirudinea are very closely related. 



Order POLYCHAETA 

 Marine Chaetopoda with numerous chaetae arising from special 

 prominences of the body wall called parapodia ; usually with a distinct 

 head which bears a number of appendages; nearly always dioecious, 

 with gonads extending throughout the body and external fertilization ; 

 with a free-swimming larva, the trochosphere. 



The structure of the Polychaeta is very variable and dependent on 

 the habit of life, both externally (especially the head appendages and 

 parapodia) and internally (especially the segmental organs). The 

 variation in methods of reproduction is also very characteristic. For 

 these reasons an account will first be given of some of the very large 

 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. 



'^Eunicidae. Eunice^ Leodice 

 (the Palolo worm). 



Nereidae. Nereis. 



Syllidae. Syllis, Myri- 

 anida. 



Phyllodocidae. Eulalia, 

 Aster ope. 



Polynoidae. Aphrodite, 



. Lepidonotus, Panthalis. 



Chaetopteridae. Chaeto- 



pterus. 

 Terebellidae. Terebella, 



Amphitrite. 

 Serpulidae. Pomatoceros, 



Filigrana. 

 Sabellidae. Sabella, Spiro- 



gr aphis. 



Arenicolidae. Arenicola 

 without jaws. 



Glyceridae. Glycera with 

 jaws. 



The errant Polychaeta with unmodified 

 head and armed eversible pharynx 

 (proboscis); 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 micro- 

 scopic 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 errant Polychaeta 

 The external structure is known to the elementary student through 

 the type Nereis (Fig. 184). The prostomium bears two kinds of 



