294 THE AMERICAN NATURALIST [Vol. XLYITI 



taken place posterior to the gonad. It was held in posi- 

 tion as previously described and had several additional 

 coils. 



Individual Variations 

 To all outward appearances any two Thyone are as 

 much alike as two peas. It was not until the internal 

 organs were studied that important differences were ob- 



served. While there are numerous minor differences, 

 the most conspicuous variations are found in the num- 

 ber, size and location of the Polian vesicles (cf. Fig. 5), 

 and in the number and arrangement of the retractor 

 muscles. On account of the radial structure of Thyone 

 not more than four Polian vesicles are present, since 



