276 



EMBRYOLOGT 



In Mitraria we recognized, a larva which possesses pro- 

 visional larval appendages in the form of long bristles, 



Fig. 124.— Lateral views of Mitraria larvte (after Mbtschnikoff, from Baifoue's 

 Comimrative Einhryology). an, anus ; b and br, the lateral elevations with the pro- 

 visional setsB; m, mouth; pr. b, preoral ciliated band; sg, apical plate. 



which also occur in other Annelid larvae. Trochophore larvas 

 are known which exhibit a number of long denticulate 

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



Sahellaria, Spio, etc. Figs. 

 125 and 126 show larvce 

 more advanced in develop- 

 ment with richly developed, 

 and in part extraordinarily 

 long, provisional setae. .SetaB 

 of this kind do not appear 

 in adult recent Chsetopoda, 

 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 Chaetopoda. This suggestion appeared to 

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

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



Fig. 125. — Larva of NtTuie (alter Alex. 

 Agassiz, from Balfodb's Comparative 

 Embryology). 



