276 



EMBRYOLOGT 



In Mitraria we i-ecognized a larva whicli possesses pro- 

 visional larval appendages in the form of long bristles, 



Fig. 124.— Lateral views of Mitraria larvfe (after Metschnikoff, from Balfour's 

 Com-parative Emhryology). an, anus ; b and br, the lateral elevations with the pro- 

 visional set»; m, mouth; pr. h, preoral ciliated band; sg, apical plate. 



which also occur in other Annelid larvae. Trochophore larvfB 

 are known "which exhibit a number of long denticulate 

 bristles on both sides of the body, thus, e.g., in the genera 



Sabellaria, Spio, etc. Figs. 

 125 and 126 show larva? 

 more advanced in develop- 

 ment with richly developed, 

 and in part extraordinarily 

 long, provisional setfB. Setae 

 of this kind do not appear 

 in adult recent Chajtopoda, 

 but, on the other hand, are 

 found in fossil forms. It 

 has been conjectured that 

 they might have been inherited from unsegmented ancestors 

 of the existing Chastopoda. This suggestion appeared to 

 be supported by the fact that they are mostly found on the 

 anterior unsegmented part of the larva (Alkx. Agassiz). 



Fig. 125. — Larva of Ntrinc (alter Alkx. 

 Aga-BSiz, from Balfoub'b Comparative 

 Embryology). 



