70 



The effect of the Parasites on their Hosts. 



extent upon some internal law of growth and not directly upon the time of year, because 

 exceptional crabs which reach 20 — 20 mm. in the winter, assume the full sexual form, though 

 uselessly, and on the other hand males which attain a medium-size in the spring pass into 

 the non-sexual vegetative phase. 



It is necessary to take these various phases into consideration, because the existence 

 of this suppressed sexual condition in the normal history of the males renders the parasitic 

 effect less striking than it would otherwise be. Yet it is sufficiently striking, a most pronounced 

 effect being visible on the abdominal length and breadth, the development of the chela, and of 

 the abdominal appendages. I will exhibit first the effect on the abdominal length. The 

 black line in Diagram 2 refers to uninfected males, the red line to infected males. It will be seen 

 at once that the average abdominal lengths associated with particular carapace lengths are in 

 all cases more than a millimeter greater in infected than in uninfected males. But a reference 

 to the Tables 3 and 4 in the Appendix on p. 96 gives an even more certain idea of the remarkable 

 modification undergone by the abdomen, for here the extreme variants are noticeable, some of 

 which have abdomina typically female in dimensions. One or two variously modified males are 

 illustrated on Plate 7 figs. 9, 10, 11, 12, and these drawings show that the abdomen under the 

 influence of the parasite may assume all the intermediate conditions between male and female. 



To sum up with regard to the abdomen, where the crab shows the character- 

 istic modification, the abdomen takes on a female character to a greater or 

 less degree, becoming long, broad and trough-like; and this occurs to an 

 easily appreciable extent in 145 out of 224 infected males examined. 



We now turn to the development of the chela (Diagram 3) and the first thing to note 

 is the characteristic deviation in the black line of growth for uninfected males between 15 and 

 18 mm. carapace length, due to the phenomenon of sexual suppression noted above. Despite 

 this, the difference between the two lines for infected (red) and uninfected (black) individuals 

 is easily discerned. The effect of the parasitism, then, is to very sensibly lessen the 

 swollen nature of the male chela and to make it approximate to the flat female 

 type which never exceeds 3 millimeters in breadth. 



In considering the abdominal appendages it must be remembered that the normal 

 males have a pair of stout copulatory styles on the first segment, followed by a rudimentary 

 appendage on the second segment (Plate 7 fig. 13). The effect of the parasitism is exhibited 

 by the tapering out or complete reduction of the copulatory style and in some cases by the 

 development of more or less rudimentary female appendages on the segments which normally 

 bear none. I have arranged the following numerical categories: 



1 = One or more female appendages present (Plate 7 fig. 11). 



2 = Copulatory style rudimentary, no Q appendages present. 



3 = Style very slender or reduced. 



4 = Style slightly reduced or tapered (Plate 7 fig. 12). 



5 = No modification. 



