278 



EMBRYOLOGY 



g aster otrochal in members of the genera Magelona, Nerine, 

 and S2)io. In the two last-named genera there are found 

 amphiirochal larvee — i.e., such as possess dorsal as well as 

 Tentral half-rings^in addition to the gasterotrochal, "just as 

 atrochal and polytrochal larvse appear in the genus Terebella. 

 The polytrochal larvae sometimes appear 

 as a stage succeeding other larval types. 

 Thus those of Arenicola marma arise from 

 larvae which at first were monotrochal, later 

 became telotrochal, and finally, by the ap- 

 pearance of new ciliated rings between those 

 already present, assumed the stage of poly- 

 trochal larvse (Max Schultze). Also the 

 true polytrochal larvae — i.e., those which 

 possess only the ciliated bands, but do not 

 yet, like many other polyti'ochal larvte, ex- 

 hibit the fundaments of the setge and other 

 parts of the body of the worm — appear as the 

 stage succeeding the Trochophore. Thus we 

 have just noted a stage entirely resembling 

 a Trochophore, which pi-eceded the polytrochal 

 larva of an Ophryotrocha. This condition warrants the 

 assumption that the segmented forms are to be looked upon 

 as the younger, the unsegmented, on the other hand, as the 

 phylogenetically older. 



As may be inferred from the manifold shapes of the 

 Annelid larvas, their metamorphosis into the worm is also 

 extremely varied. 



This has already been briefly discussed in some forms 

 while considering the larval stages. The segmentation may 

 be expressed on the body of the larva in various ways. In 

 some cases the body elongates and divides into segments, 

 while the ciliated bands are still retained. In other forms the 

 setae alone first make their appearance in pairs, and indicate 

 the segmentation of the body, or at the same time the 

 parapodia are established in the form of protuberances. 

 Thus larvsB are found which have still preserved the entire 

 form of the Trochophore, and yet exhibit already the two 

 lateral rows of setae or pai'apodia. At first only a few 



Fig. 127.— Poly- 

 trochal larva of 

 Oplu-ijoirocha pue- 

 rilis (after Clapa- 



BEDE UND MeTSCH- 



nikoff). d, intes- 

 tine ; fc, jaws. 



