No. 3.] THALASSEMA AND ZIRPHAEA. 627 



the origin of these thickenings, we do not Icnow whether they 

 are homologous with the loops of Zirpkaea or the spheres of 

 an ordinary tetrad. Bolles Lee ('97) figures and describes a 

 type in Helix in which tetrads arise from rings, but later 

 become compacted into solid chromatin masses. His figures 

 suggest the possibility of a rotation whereby the spheres are 

 halved. 



It seems not impossible that the mode of division in Thalas- 

 sema and other forms characterized by persistent ring chromo- 

 somes may vary in some degree. We have seen that in cases 

 where the ring fails to loop, it may become attached to the 

 fibers in varying ways. If we suppose the attachment to be 

 fixed and to fall anywhere except in the normal division planes, 

 the result will be a division in planes more or less inclined to 

 the normal ones.^ Weismann ('91) has especially urged this 

 possibility in ring-form chromosomes. 



I have found evidence of a type more or less similar to that 

 of Thalassema or Zirpkaea in Teredo, Pholadidea, and Nereis. 



Zoological Laboratory, Columbia University, 

 March, 1898. 



* Persistent ring chromosomes have been described or figured in Plants 

 (Farmer and Moore), Platodes (v. Klinckowstrom), Elasmobranch (Moore), Mouse 

 (Sobotta). 



