CH^TOPODA. 



329 



between a large mouth-bearing portion of the primitive body 

 and a small anus-bearing portion 1 . 



The general mode of development of Chsetopod larvae is 

 similar to the above except in details, which are however no 

 doubt often of great importance. The history of the larvae may 



FIG. 148. POLYGORDIUS LARVA. (From Alex. Agassiz.) 



be conveniently treated under three heads, (i) The form of 

 the primitive unsegmented larva; (2) the arrangement of the 

 cilia on the unsegmented larva, and on the larva at later stages ; 

 (3) the character of the metamorphosis and the development of 

 the permanent external organs. 



A larva similar to the Polygordius larva with a greatly 

 developed prae-oral lobe is widely distributed amongst the 

 Annelids. 



An almost identical form is that of Nepthys 

 scolopendroides (Claparede and Metschnikoff, No. 

 336) ; that of Phyllodoce (fig. 149) is also very 

 similar, and that of Saccocirrus (Metsch. and Clap. 

 No. 336, PL XIII. fig. i), a very primitive form most 

 nearly related to Polygordius, clearly belongs to the 

 same type. Many other larval forms, such as that 

 of Spio fuliginosus (Metsch. and Clap. No. 336), Tere- 

 bella, Nerine, etc., also closely approach this form. 



FIG. 149. LARVA OF 

 PHYLLODOCE. (From 

 Alex. Agassiz.) 



Other really similar forms at first sight 

 appear very different, but this is mainly 

 owing to the fact that their prae-oral lobe never attains a 

 considerable development. Its smallness, though obviously of 

 no deep morphological significance, at once produces a very 

 different appearance in a larva. 



1 For Semper's view as to the intercalation of segments in the cephalic region, 

 vide note on p. 333. 



