EUPLECTELTvA. 57 



latter into the former. And yet, in view of the indubitable 

 cases of oxyhexasters reverting back again into the more primary 

 bexactin-shape (hexactinose oxybexaster), a somewhat analogous 

 retrogression of discobexasters into oxyhexasters may be said to 

 be not altogether impossible to imagine. However, if this really 

 takes place at all, it must be of casual occurrence and would re- 

 quire special circumstantial evidence in order to be recognized 

 as such. The presence of the incipient forms of discobexasters, 

 i.e., the onychasters, among isolated members of different families 

 (Euplectellidse, Melittionidse) would probably require for its 

 explanation an assumption of its independent origination at 

 separate points in the phylogeny by convergent adaptation. 



In all thirteen species of Eiupleetella are to be considered 

 as known at present. I will on tlie next page annex a key to all 

 these species, whicli should bring out the main points of their 

 structural differences and indicate to some extent the affinities 

 existing among them. 



