No. 6.] PARASITISM OF ANODOA'TA PLANA LEA. 303 



Moreover, the color is not indiscriminately scattered as if 

 merely accidentally, but it is very definite in its distribution in 

 two respects ; namely, in the single valve and in both valves. 



First in one valve it appears to be a law that the location of 

 the coloration varies with the amount of the red material. 

 Thus (a] in case of a very small deposit this is found directly 

 beneath the umbo of each valve and in no place else ; (b) in 

 case of a larger deposit the color is found in a patch which 

 extends from the umbo downwards and forwards towards the 

 ventral border of the shell ; (c) in case of still more red there 

 is a second narrower patch 

 running down posteriorly 

 from the umbonal area to- 

 ward the ventral border of 

 the shell ; (</) in cases of 

 still more coloration these 

 areas are filled in between 

 so that in some cases the 

 red crosses the shell above, 

 but not ventrally, and in 

 others it extends to the 

 pallial line, or even beyond this to the very margin of the shell 

 (see Fig. i). 



It will be noted that these differences are not progressive 

 stages observed on a single individual, but they are different 

 cases that are observed. They may, however, indicate a prog- 

 ress in the growth of the coloration ; if so, then we should 

 have to say that the process begins at the umbo, then spreads 

 in a narrow line downwards and outwards, first anteriorly, then 

 posteriorly, and that later it spreads over the whole shell from 

 these points. While I do not know of any actual evidence 

 to show that such is the case, still the facts indicate some 

 such order. 



Not only is the coloration of the single valve thus definite, 

 but it is an even more remarkable fact that the effect is uni- 

 form on the two valves ; that is, the amount and position of 

 the coloration is always bilaterally symmetrical. If one valve 

 has a small spot at the umbo, then so has the other; there is 



FIG. i. Inner surface of valve of Anodonta. The 

 dotted lines indicate areas affected by the color- 

 deposit. 



