278 



EMBRYOLOGY 



giisterotrochal in members of tlie genera Magelona, Nerine, 

 and Spio. In the two last-named genera there are found 

 ampMii-ochal larva? — i.e., such as possess dorsal as well as 

 ventral half-rings — in addition to the gasierotroclial , just as 

 atrochal and polytrochal larvae appear in the genus Terebella. 

 The polytrochal larvae sometimes appear 

 as a stage succeeding other larval types. 

 Thus those of Arenicula marina 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 larvae (Max Schulize). Also the 

 true polytrochal larvfe — i.e., those which 

 possess only the ciliated bands, but do not 

 yet, like many other polytrochal larvos, ex- 

 hibit the fundaments of the setae 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 TrochopJiore, which preceded 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 larvae, 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 larvae are found which have still preserved the entire 

 form of the Trochophore, and yet exhibit already the two 

 lateral rows of setse or parapodia. At first only a few 



Fig. 127.— Poly- 

 trochal larva of 

 Oi)hryoiroeha jiwe- 

 rilis (after Ci.apa- 



BEDE UND MeTSCH- 



nikoff). d, intes- 

 tine ; fc, jaws. 



