CH.ETOPODA. 



277 



a division into notopodium and ueuropodium; 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 be- 

 fore there are any other visible rudi- 

 ments of the feet (e.g. Lumbriconereis) ; 

 while in other cases the reverse holds 

 good. The gills are usually the last 

 parts to appear. 



Not only does the mode of develop- 

 ment of the feet differ greatly in 

 different types, but also the period. 

 The appearance of set may afford the first external indication of 



FIG. 152. LAKVA OF NEKINE, 



WITH PROVISIONAL SET/E. (From 



Alex. Agassiz.) 



FIG. 153. EMBRYO CH-ETOPOD WITH PROVISIONAL SET^E. (From Agassiz.) 



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 are usually 

 placed at the sides of the anterior part of the body, immediately be- 

 hind the head, and also sometimes on the posterior parts of the body. 

 In some instances (e.g. fig. 153) they form the only appendages of the 

 trunk. Alex. Agassiz has pointed out that setae of this kind, though 

 not found in existing Chaatopods, are characteristic of the fossil 

 forms. Setae of this kind are found in chsetopod-like larvae of some 

 Brachiopods (Argiope, fig. 13G). 



It is tempting to suppose that the long provisional bristles 

 springing from the oral region are the setiform appendages handed 

 down from the unsegmented ancestors of the existing ChaBtopod 

 forms. Claparede has divided Chaetopod larvae into two great groups 

 of Metachaetae and Perennichgetae, according as they possess or are 

 without provisional setae. 



With reference to the head and its appendages it has already 

 been stated that the head is primarily formed of the prae-oral lobe 

 and of the peristomial region. 



The embryological facts are opposed to the view that the prae-oral 

 region either represents a segment or is composed of segments equi- 



