go The effect of the Parasites on their Hosts. 



situated on the second, third, fourth and fifth segments. No intermediate state between the 

 adolescent and adult crabs exists, i. e. the two forms are separated by a single ecdysis. 



The full reproductive season begins in March in the Bay of Naples and goes on con- 

 tinuously till August, two broods at least being produced by every crab. The characters to 

 which I have paid attention are the abdominal length and breadth, the length and breadth of the 

 chela, and the abdominal appendages, since these are the characters signally different in the two 

 sexes. These characters will be considered in relation to the carapace length which is taken as an 

 index of general growth, since it has been determined that growth in length, breadth and depth of 

 the carapace is practically proportional at all ages and in both sexes whether infected or uninfected. 



The first character to be considered is the abdominal length; my results for the 

 breadth are not given because they are substantially the same. Diagram 1 (p. 71) is a diagram 

 composed from Tables 1 and 2 in the Appendix' which refer to the rate of growth of unin- 

 fected and infected females respectively, in abdominal length. The Diagram is constructed by 

 drawing a line as nearly as possible through the average or mean condition of each abdominal 

 length associated with each particular carapace length. The black line refers to uninfected 

 females, the red to infected females. It is seen at a glance that the black line follows a diffe- 

 rent pitch before and after 12 millimeters carapace length, and this is due to the distinction 

 already drawn between the abdomina of adolescent and adult females. The remarkable fact 

 disclosed by comparing this black line with the red line referring to infected females is that 

 the result of infection has been to straighten out the curved line of growth, and this is due 

 to the fact that the small infected females under 13 millimeters no longer appear as a rule 

 under the adolescent type, but as adults with long broad and trough-like abdomina. This 

 result is, I believe, highly unexpected; but the fact is beyond question. 



After 13 millimeters no obvious difference can be detected between infected and unin- 

 fected females in respect to the abdomen. 



The effect then of the parasitism on young females under 13 mm. is to make 

 them prematurely assume the adult type of female abdomen. This is of great 

 theoretical interest because it helps to preclude the possibility of interpreting the effect of 

 the parasite as being a return to an embryonic, sexually undifferentiated condition. 



The chela length and breadth is uninfluenced by the parasitism. 



With regard to the abdominal appendages I have arranged five categories for in- 

 fected females as follows: 



1 . appendages rudimentary or absent, 



2. appendages much reduced Plate 7 fig. 5), 



3. endopodites only of appendages absent or much reduced, 



4. endopodites slightly reduced, 



5. appendages wholly unmodified 



Table 3 Appendix p. 96) sets out, I believe with substantial truth, the effect of the parasi- 

 tism upon this character in females of various sizes. From this it appears that 55 out of 245 are 



