334 



LARVAL FORMS. 



FIG. 151. LARVA OF 

 PHYLLODOCE FROM THE 



VENTRAL SIDE. (From 



Alex. Agassiz.) 



Apart from the formation of the segments the larval meta- 

 morphosis consists in the atrophy of the 

 provisional ciliated rings and other provi- 

 sional organs, and in the acquirement of 

 the organs of the adult. 



The great variations in the nature of the 

 Chaetopod appendages render it impossible 

 to treat this part of the developmental 

 history of the Chaetopoda in a systematic 

 way. 



The mode of development of the append- 

 ages is not constant, so that it is difficult to 

 draw conclusions as to the primitive form 

 from which the existing types of appendages 

 are derived. 



In a large number of cases the primitive rudiments of the 

 feet exhibit no indication of a division into notopodium and 

 neuropodium ; while in other instances (e.g. Terebella and Nerine, 

 fig. 152) the notopodium is first 

 developed, and subsequently the 

 neuropodium quite independently. 



In many cases the setae appear 

 before there are any other visible 

 rudiments of the feet (e.g. Lumbri- 

 conereis) ; while in other cases the 

 reverse holds good. The gills are 

 usually the last parts to appear. 



Not only does the mode of 

 development of the feet differ 

 greatly in different types, but also the period. The appearance 

 of setae may afford the first external indication of segmentation, 

 or the rudiments of the feet may not appear till a large number 

 of segments are definitely established. 



A very considerable number of Chaetopod larvae are provided 

 with very long provisional setae (figs. 152 and 153). These setae 



of the interpolation of the head segments, bearing the gills, in Terebella appears to 

 me quite unjustified from Milne-Edwards's own statements ; and is clearly shewn to 

 be unfounded by the careful observations of Claparede on Ter. conchilega, where the 

 segments in question are demonstrated to be present from the first. 



FIG. 152. LARVA OF NERINE, 



WITH PROVISIONAL SET^E. (From 



Alex. Agassiz.) 



