58 



MARINE BIOLOGICAL LABORATORY. 



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brates indicate real affinity, then the trochophore larva 

 must be taken to represent, in a certain sense, the an- 

 cestor of all the higher forms of life. According to a 

 , second school, however, the 



trochophore has no such re- 

 markable significance, but is 

 a ** secondary" or ''adaptive" 

 larval form — i.e., one second- 

 arily interpolated into the de- 

 velopment and representing 

 no ancestral group ; as is the 

 case, for example, with the 

 larvae of insects. Whichever 

 view be taken, a precise knowl- 

 edge of the trochophore is 

 essential to the investigation 

 of the general problems indi- 

 cated; and I shall show further 

 on that the study of this re- 

 markable larva raises a num- 

 ber of very singular questions 

 regarding the nature and origin 

 of the higher forms of life. 



I trust that this introduction 

 will suffice to make clear the 

 general considerations which, 

 in my opinion, render the annelids, and particularly their 

 trochophore larvae, worthy of especial attention ; and 

 which give value to the investigation of every detail of 

 their morphology. We turn now to a more special 

 account of the annelids. 



The body of an annelid (Fig. i, which may be taken 



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Fig. I. 



