2. Effect on the primary and secondary sexual characters. g9 



unmodified; of these 40 are over 15mm. in carapace length. That is to say about 1/1 th of 

 the crabs under 15 mm. are unmodified, while nearly 1/2 over 15 mm. are unmodified. The 

 whole table indeed shows that the proportion of modified crabs and the degree of modification 

 in the abdominal appendages increases as we proceed to younger and younger crabs. This 

 holds good except in those very youngest crabs which have the adolescent type of abdomen 

 and appendage which does not appear to be so liable to modification as the adult. To sum 

 up, it appears that the parasitism of Sacadina on the secondary sexual characters of females 

 of Inachus scorpio has a definite effect which chiefly consists in the reduction of the abdominal 

 appendages, but except in this purely negative character the infected females do 

 not show any approach to the male structure. 



2. Males. There is a curious fact with regard to the growth and development of the 

 secondary sexual characters in normal uninfected males of /. scorpio, which will be only touched 

 on here, as elsewhere (see p. 90) I have treated the matter more fully. It is that during the 

 breeding season the males appear under three chief types, firstly small males under 10 mm. 

 with rather swollen chelae (Plate 7 fig. 8), secondly males of medium size 1 with flat female- 

 like chelae (Plate 7 fig. 7), and thirdly males from 20 — 26 mm. with enormously swollen chelae 

 (Plate 7 fig. 6). It is true that a few medium sized crabs with swollen chelae can be found in 

 summer which form a link between the first and third categories, but these are very rare ; indeed 

 the whole number of crabs belonging to the second category are very rare during the breeding 

 season. The medium sized crabs with flat chelae on dissection prove to have poorly developed 

 testes and vesiculae seminales compared to the other two kinds, and spermatogenesis is in 

 abeyance. It appears therefore that only two categories of male crabs take part in the 

 breeding season, differing from one another in point of size and in the development of the 

 chelae. In winter a great number of small and medium sized crabs are met with, all with 

 flat chelae, and exceedingly few crabs of the first and third category. The explanation of this 

 is undoubtedly that young crabs of 13 — 16 mm., after taking part in the breeding season of 

 one year, pass into a kind of non-sexual state for the winter and growing rapidly appear as 

 fully developed males of the third category in the following spring. But it appears that the 

 assumption of the suppressed and of the fully developed sexual states is dependent to a large 



l ) An explanation of these medium sized crabs with flat chelae may occur to the reader in the possibility 

 of their chelae having undergone regeneration. This explanation is however quite precluded by the following facts. 

 1 . The chelae of the medium sized males are always symmetrical so that if they have regenerated, both chelae must 

 have been lost at the same time. Now in more than a thousand males only ten have been found in which one 

 chela was plainly a regenerated one, but the chance of a crab losing both chelae at once is much smaller than 

 the chance of its losing one. 2. Why should the medium sized crabs with flat chelae always have poorly developed 

 testes if the reduction of their chelae is simply due to accidental loss and regeneration? 3. Why should males 

 with flat chelae and poorly developed testes occur normally in the winter when males with swollen chelae are very 

 rare, especially as in the winter the males do not fight for the females and so are less liable to lose their chelae? 

 4. Why should the flat chelae be associated with males of a certain size, when breeding males, liable to the loss 

 of chelae, are of all sizes except of this particular size with flat chelae? 



