CKLETOPODA. 335 



are usually placed at the sides of the anterior part of the body, 

 immediately behind the head, and also sometimes on the 

 posterior parts of the body. In some instances (e.g. fig. 153) 



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



they form the only appendages of the trunk. Alex. Agassiz has 

 pointed out that setae of this kind, though not found in existing 

 Chaetopods, are characteristic of the fossil forms. Setae of this 

 kind are found in chaetopod-like larvae of some Brachiopods 

 (Argiope, fig. 136). 



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 

 Chaetopod forms. Claparede has divided Chaetopod larvae into 

 two great groups of Metachaetae and Perennichaetae, 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 

 prse-oral region either represents a segment or is composed of 

 segments equivalent to those of the trunk. The embryonic 

 peristomial region may, on the other hand, be regarded as in a 

 certain sense the first segment. Its exact relations to the 

 succeeding segments become frequently more or less modified in 

 the adult. The prae-oral region is in most larvae bounded 

 behind by the ciliated ring already described. On the dorsal 

 part of the prae-oral lobe in front of this ring are placed the 

 eyes, and from it there may spring a variable number of 

 processes which form antennas or cephalic tentacles. The 

 number and position of these latter are very variable. They 

 appear as simple processes, sometimes arising in pairs, and at 



