250 bobert h. wolcott: 



impaired — indeed, it is quite as good a swimmer as A. crassipes. 

 Aside from increase in size and unwieldiness of form, there is 

 a lack of the bright coloring which makes the whole group such 

 interesting objects of study. The method of oviposition leads 

 finally to peculiarities of structure in connection with the 

 female sexual organs, consisting of the spines about the genital 

 opening previously referred to. If kept for some time in a 

 vessel of water cannibalism is never resorted to, no food is 

 taken, and after weeks of starvation the mites finally die. 



Haldeman (42) says they do not suffer from cold, as they are 

 pretty active in water a few degrees above the freezing point; 

 and he has found them moving about in a [fnio, the outside 

 layer of which was frozen. Nevertheless, they become torpid 

 instantly, he says, if placed in freezing water, though the tor- 

 pidity remains but a short time, if the temperature be gradually 

 raised. As a rule, the migrations of the mussel from shallow 

 to deep water and its habit of burying itself in mud during the 

 winter, must make the temperature conditions within the shell 

 very constant. Equally uniform is the supply of food, which 

 is furnished to the animal with a minimum amount of exertion. 



This stability of conditions under which the animal lives 

 would lead us to expect little variation among the mites and 

 little is found. Cases occur, involving the number, size, form, 

 etc., of acetabula on the genital plates, and to a slight degree 

 the relative dimensions and characters of other parts. But in 

 no case are these sufficient to cause any confusion amongst 

 different species, even when these are found living together in 

 the same mussel. Specific characters are well defined and con- 

 stant, and with care in the examination of specimens and a 

 knowledge of the characteristic structural features of each 

 species, identification should be perfectly sure. 



The strange thing is that under conditions so stable, so 

 many species should occur. In this respect as compared with 

 the European fauna ours seems remarkably rich, since we have 

 now 13 recorded species of Atax as compared with 8 from all 

 parts of the continent of Europe, and our list is likely to be 

 increased by collections in the more distant parts of the coun- 



