in the Common Shore-crab caused by Sacculina. 99 



The male crab with the largest index does not approximate 

 very closely to the female type, and those so markedly modified 

 are but a small proportion of the whole. A third of the crabs 

 measured could be called entirely unmodified, a proportion which 

 corresponds roughly with the quarter unaffected by the parasite 

 in the case of the spider-crab, and of the hermit-crab. 



The number of crabs measured was 59, so that the series is 

 too small to be satisfactorily plotted in the form of a curve. Some 

 idea of the distribution may be gained by the statement that 

 37 possessed an index between "56 — '66, while that of the re- 

 maining 22 lay from "67 to '77. In the first group there were 

 11 wdth an index of "60, and in the second 5 with one of "72. 



One point in Giard's short account which I desired to test was 

 his assertion that the older (or larger) crabs were not subject to 

 modification. The average carapace breadth of the crabs of index 

 •56 — '66 to be sure is 4*9 as compared to 4*5 of those from 

 •67 — ^77, but the figure for the index -72 is 4-6 and for -77 the 

 extreme limit of modification is 4"7. Occasional crabs of excep- 

 tional size are met with, with abdomina considerably broadened, 

 and my observations give practically no support even for the 

 statement that younger crabs are more liable to modification than 

 older individuals. 



In the abdomen the effect of the parasite was distinctly less 

 than is the case with the spider-crabs. With the other secondary 

 sexual characters no alteration was experienced. There was no 

 diminution of the copulatory styles in size and no appearance 

 of the abdominal swimmerets which are characteristic of the 

 female. 



In the female no change was ever detected in the abdomen as 

 a result of parasitism, and the swimmerets appeared fully as 

 well-developed and thickly fringed with hairs as in crabs free 

 from the Sacculina. 



With regard lastly to the sexual glands of the infected shore- 

 crab, here again there is little alteration. The females never bear 

 eggs, and the ova in the ovary are as a rule small and white, and 

 devoid of great stores of yolk. In the uninfected male the testes 

 are lobulated organs, stretching on each side across the liver. 

 The development varies greatly even in normal specimens. The 

 vasa deferentiae follow, and speedily become wide tubes with a 

 very characteristic milky white appearance, due to the multitude 

 of spermatophores, each crammed with spermatozoa, which they 

 contain. Posteriorly the male duct opens on the terminal joint 

 of the last thoracic leg, at the end of a long fleshy penis. 



In infected males a certain diminution in size of the testis is 

 often noticeable, but actual atrophy never takes place. There is 

 very little reduction in size of the vasa deferentiae, and they always 



