262 



POLYCHAETA 



CHAP. 



The tentacles in the Nereidiformia present a wide variation 

 in number ; probably the typical number is three, one of which 

 is median and two lateral as in Polynoids, Syllidae, and some 

 Eunicidae. Further, there is a certain amount of evidence in the 

 nerve supply of the median tentacle to show that it was origin - 



e d 



Fig. 134. Heads of various Polychaeta (diagrammatic). A, Polynoid ; B, Syllid ; C, 

 Nephthys ; D, Eunice ; E, Phyllodoce ; F, Trophonia : a, prostomium ; c, normal 

 cirrus ; c 1 , peristomial cirri ; c'\ cirrus of second segment ; c 3 , cirrus of third seg- 

 ment ; el 1 , point of attachment of elytron ; p, palp : s, nuchal organ (ciliated pit) ; 

 t, tentacle ; I, peristomium ; II, III, IV, segments. 



ally double. The presence of four tentacles, then, as in Nephthys, 

 Phyllodoce, and Glycera, may be a primitive condition. By the 

 disappearance of the paired lateral tentacles the worm possesses 

 a single median one, as in Aphrodite and Amphinomids; 1 whilst a 

 duplication of these lateral ones leads to the condition of Eunice 

 and Hyalinoecia, which have five tentacles. In the Chlorhae- 

 midae the number is further increased to five or more on each 



1 In some of the members of this family paired lateral tentacles appear to exist. 



