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 



FIG. 122. Nereis pelasjica organ 

 L., natural size. (From 

 Elilers. ) 



name " parapodium " 



(Fig. 124). Each para- 

 podium or foot consists of a basal portion, 

 supporting a dorsal and a ventral process, the 

 "notopodium" {ntp} and "neuropodium"(?t?y>) 

 respectively, each of which is bilobed. The 

 lobes are very vascular and glandular, and 

 probably serve as respiratory organs or "gills." VlG - r !23. 



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 

 long stalk and a small " appendix " articulated in a cup at its 



Chaetae of 

 enlarged. A, 

 from neuropodium ; B, 

 from iiotopodium of Jf. 

 diversicolor ; C, swim- 

 ming chaeta of Hetero- 

 nereid stage of N. 

 dinner ilii. 



