6o 



MARINE BIOLOGICAL LABORATORY. 



nation of body to head in studying the development of 

 an annelid. 



Upon this simple plan all annelids are constructed, 

 though they are almost as varied in the details of their 

 orofanization as the vertebrates themselves. The head 

 may be provided with the most elaborate lobes, tentacles, 

 cirrhi, branchiae, eyes, etc. ; or it may lose all of these 

 special organs and become reduced to an insignificant 

 rudiment, as in the earthworm. The trunk is no less 

 diversified ; sometimes a simple jointed cylinder, some- 

 times provided with lateral appendages of the most 

 diverse character in different forms and performing 

 many functions — locomotion of many varieties, respira- 

 tion, sensation, etc. 



Let us now turn to the development ; I select Poly- 

 gordms, a form generally regarded as one of the sim- 



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Fig. 2. 



plest and most primitive of the annelids. Polygordins 

 has a perfectly typical trochophore larva (Fig. 2), shaped 



