NORTH AMERICAN SPECIES OF THE GENUS ATAX 249 



presence causes the growth of pearly prominences on the inside 

 of the shell, and it is possible the irritation due to the presence 

 of eggs in the mantle, might in some cases lead to this. The 

 figures collected the past summer are not sufficient to allow of 

 accurate statements being made. When the attempt is made to 

 allow for the effect of age, of variation in form in the different 

 species, and situation as affecting the probability of the en- 

 trance of foreign particles, the number of mussels examined 

 does not furnish sufficient data for definite conclusions. Other 

 sources of uncertainty are added when it is considered that 

 even if no mites are found in a mussel of considerable size the 

 assumption that it never has contained any is entirely unwar- 

 ranted, and when it is remembered that in case of infested 

 mussels we have no means of knowing how long the mites have 

 been there and whether or not sufficient time has elapsed for 

 the demonstration of any effects on the host. 



But upon the mites themselves the effect is more pronounced. 

 Living in the gills during development and later in a slimy 

 secretion, as they do, the tracheal system is less perfectly 

 developed, in some cases becoming quite rudimentary. Yet 

 they live for weeks in a vessel of water after removal from 

 the mussel. Here the other effects of parasitism become 

 apparent, especially if we compare free-living and parasitic 

 species. The body of the parasitic forms is large and more or 

 less unwieldy, in the case of the females of A. twmidus and 

 A. (JSf.) ingens so large, at times, as to incapacitate the mites 

 for any movement, and the great thing lies on its back kicking 

 its legs in the air utterly unable to even hold itself right side 

 up. The swimming powers are feeble and the animal sinks at 

 once to the bottom where it clambers clumsily about, fortunate 

 if any soft object allows it to obtain a foothold, at a great dis- 

 advantage if it be on a bottom too hard to allow its claws to 

 penetrate. An apparent immunity from this effect of parasit- 

 ism is seen in the case of A. aculeatvs which, living as it does 

 along the edge of the mantle and about the exhalent aperature, 

 is more exposed to currents of water and retains the length of 

 legs, small size of body and activity of movement almost un- 



