246 



POLYCHAETA 



colour is brown or bronze. The worm, which is to be found in 



shallow water, is made up of a considerable number of rings or 

 segments, constituting the "trunk" or "body," 

 terminated at each end 

 by modified segments 

 known as " head " and 

 "tail "(Fig. 122). The 

 segments composing the 

 trunk are all alike, ex- 

 cept for small propor- 

 tional differences, and it 

 will be convenient to 

 describe a " typical seg- 

 ment " before referring 

 to the head or tail. 



A typical body seg- 

 ment carries on each side 

 a muscular lobed out- 

 growth, bearing bundles 

 of bristles or " chaetae," 

 and filamentous sensory 

 organs known as " cirri." 

 To this lateral locomotor 

 organ Huxley gave the 

 name " parapodium " 

 (Fig. 124). Each para- 

 podium or foot consists of a basal portion, 

 t supporting a dorsal and a ventral process, the 



"notopodium" (ntp) and "neuropodium"(wrp) 



respectively, each of which is bilobed. The 



lobes are very vascular and glandular, and 



probably serve as respiratory organs or "gills." 

 The chaetae, or bristles, of each bundle 



project from the mouth of a great sac, the 



lips of which are particularly prominent in 



Nereis. Each chaeta arises from a single 



cell situated at the bottom of the sac. The 



chaetae of Nereis, as of many other Polychaetes, are of a kind 



usually termed compound or " jointed," each being composed of a 

 a small " appendix " articulated in a cup at its 



Fig. 122. Nereis pelagica 

 L., natural size. (From 

 Ehlers. ) 



Fig. 123. Chaetae 

 Nereis ; enlarged, 

 from neuropodium ; B 

 from notopodium of N, 

 diversicolor ; C, swim 

 ming chaeta of Hetero 

 nereid stage of N. 

 dumerilii. 



long stalk and 



