2. Effect on the primary and secondary sexual characters. 73 



C. Crabs of apparently doubtful sex. 



I have studied altogether about 200 infected crabs which had. been so completely 

 modified both externally and internally that it was impossible at first sight to assign them 

 with any certainty to either sex. All of them had the female type of abdomen, but all of 

 them possessed a pair of copulatory styles in more or less perfection, and all of them had 

 the characteristic female appendages, either perfectly or imperfectly developed (Plate 7 tigs. 14, 

 15, 16, 17). The chela was in all cases but one of the female type. All the specimens were 

 small, most being 9 — 11 mm., while only one was over 15 mm. (Plate 7 figs. 18, 19). 



It appeared to me most important to determine the state of the gonads in these crabs. 

 I was therefore not content with dissection but in numerous cases made preparations and serial 

 sections of the whole contents of the thorax. 



In the great majority of these crabs 1 was unable to find the least trace of a gonad, 

 and yet in infected crabs of the same size which showed by their secondary sexual characters 

 that they were either male or female, I never failed to detect a differentiated gonad, though 

 often much reduced. 



In three infected crabs which were externally perfect hermaphrodites, I was able to 

 detect the presence of much reduced vesiculae seminales with exceedingly fine strands of 

 tissue passing upward from them, which were doubtlesS the remains of testes. These three 

 crabs were therefore all males, and for a variety of reasons I am forced to the conclu- 

 sion that all these crabs of apparently doubtful sex are in reality highly modified males. 

 Perhaps the most cogent reasons for this conclusion are given under the cases of Pachygrapsus 

 marmoratus and Eriphia spinifrons further on, but a consideration of the facts in 1. scorpio 

 alone are sufficient almost to convince us. For it has been seen in Paragraph A that infected 

 females, as regards the chela and the abdomen, do not in any degree approach the male form, 

 but that infected males in both these characters and also in the presence of female appendages 

 become assimilated in all their characters to the female form. Thus a perfect transition in 

 every detail can be traced from undoubted males to these crabs of doubtful sex, while 

 females, since they never assume any other male characteristic under infection, can hardly be 

 supposed to suddenly develope the typically male copulatory styles. Further the great majority 

 of these crabs are found in October and November, and among the whole number of infected 

 individuals only a very small percentage can be recognised as males, the rest being either clearly 

 female or else hermaphrodite. If therefore the latter are not really modified males, we must 

 suppose that a very much greater percentage of females are infected than males, which is contrary 

 to all the observations I have made on this head. Finally, since in the three cases in which 

 the remains of a gonad could be detected, the gonad was male in structure and histological 

 characters, though containing no sperm, I conclude that all these crabs of apparently doubtful 

 sex are really highly modified males. 



There are thus two conclusions from the study of these crabs to be specially emphasised. 



Zool. Station zu Neapel, Fauna und Flora, Golf von Neapel, Rhizoeephala. 10 



