EXTERNAL CHARACTERS OF NEREIS. 



351 



3. Chaetae. The chaetae, as represented in Fig. 182, 

 are of two kinds one with a long slender blade or head as 

 in A, the other with a much shorter curved 

 head. In both the head is furnished 

 with fine teeth on the concave edge. The 

 head is articulated in a socket at the top 

 of the shaft, and is somewhat easily de- 

 tached. The shaft shows a number of 

 bands side by side, marked with trans- 

 verse striae. 



4. The Heteronereis or Epitokous 



Form. Many species of Nereis exhibit 



remarkable and interesting differences in 



the structure 

 of the para- 

 podia of the 

 posterior part 

 of the body 

 when they 

 become sex- 

 ually mature, 

 the append- 

 ages of the 

 anterior third 

 of the body 



Fig. 181. PARAPODIUM OF THE 

 IIETKRONKRKI9 OR EPITOKOUS 

 FOHM OF NKUEIS MAGNIFIED. 



remaining 

 unchanged. 

 The modified 

 larger and 



Fig. 182. CHAETAE OF 

 NKREIS HIGHLY 

 MAGNIFIED. 



A, Chaeta of ordin- 

 ary form with Ions; 

 blade. B, Ditto with 

 short blade. (?. Chae- 

 ta of Heteronereis or 



parapodia appear much 



broader. The differences consist in the 



development of very broad flat expansions 



on the inner lobes of the parapodium 



and at the bases of the cirri, while the 



outer lobes are little affected. The chaetae in the modified 



parapodia are more numerous and of different shape, the 



blades or heads being remarkably broad and flat, like the 



blade of an oar. All these changes make the parapodium 



(Fig. 193, c) much more efficient as a fin, and enable the 



worm to change its mode of locomotion from creeping or 



