/ ON THE NORTH AMERICAN SPECIES OF THE GENUS 

 ^ ATAX (FABR.) BRUZ. 



^ . 



"^ ROBERT H. WOLCOTT. 



Q GENERAL CONSIDERATIONS. 



-*J One who opens many of our fresh-water mussels, cannot fail 



to notice, in part of them at least, dark spots upon the mantle or 

 gills, which a moment's observation will show are living, moving 

 organisms. A lens will reveal the fact that they have four pairs 

 of six-jointed legs and a pair of five-jointed palpi, but no 

 antennae-like structures, that the head, thorax and abdomen are 

 fused into one mass with no trace of segmentation, and that 

 the relatively long legs are clothed with spines and hairs which 

 assist in swimming. Two small blackish or brownish eyes 

 may be detected near the anterior margin. These characters 

 point to the taxonomic position of these creatures in the order 

 Acarina or Mites and in the family Hydrachnidae or Water- 

 Mites, while their presence in the mussel suggests their mem- 

 bership in the genus Atax, the members of which are mussel- 

 parasites, during at least a part of their existence. Rarely 

 representatives of other non-parasitic genera may be found 

 within these shells, but their occurrence there is purely acci- 

 dental; on the other hand few species of this genus are found 

 except in mussels. However, the genus is not confined to the 

 Unionidse, a single species having been found in the mantle- 

 cavity of a South American gasteropod, Amp nil 'aria, related 

 to our genus Cmripeloma, and the author having detected 

 another previously described species in a species of Sphc&rvum, 

 one of the Cyrenidae. 



In scanning the literature on the subject we discover but 



scattered references to Hydrachnidae, under the generic term 



Acarus, previous to 1781, when O. F. Miiller described 19 



species from Denmark, establishing for them the new genus 



13 



