38 Cunningham, Unisexual Inheritance. 



Taking the words n removal of the generative organs" literally, 

 Professor Meldola's question may perhaps be answered in the nega- 

 tive ; for although artificial castration has been attempted in fish, cray- 

 fish and even caterpillars, the effects of the operation on the unisexual 

 characters do not appear to have been ascertained. It so happens, 

 however, that there exists very remarkable evidence concerning the 

 effects of a natural process of castration in one at least of the lower 

 classes of animals, namely, the Crustacea. I refer to the remarkable 

 observations which have been made principally by the eminent french 

 zoologist, Alfred Giard, on what is known as parasitic castration. 

 Professor Me Idol a, whose zoological studies have been chiefly devoted 

 to insects, appears to be unacquainted with these important obser- 

 vations and they seem to me to afford strong support to my conclusions. 

 One case which has been most carefully investigated is that of crabs 

 infested by the parasite called Sacculina. The common shore crab 

 Carcinus mo en as is frequently attacked by one] species of Sac- 

 culina. The parasite is a Cirripede, its early stages showing that 

 it is allied to the barnacles. In its adult condition it forms a conspi- 

 cuous bulbous structure attached by a thin peduncle to the lower sur- 

 face of the abdomen ; the so-called tail, of the crab. The peduncle is 

 connected internally with a system of branching roots which ramify 

 through nearly all the internal tissues of the crab. By means of these 

 roots the parasite absorbs its nourishment from its host. Crabs in- 

 fested with the parasite are almost invariably sterile. The generative 

 organs, ovaries or testes, do not appear to be destroyed, but they are 

 unable to become functionally active. The interesting fact in relation 

 to my present subject is that male crabs infested by the Sacculina 

 resemble the females externally, their secondary sexual characters 

 being to a great extent suppressed, just as the antlers are suppressed 

 in castrated stags. In the common shore crab the unisexual characters 

 of the normal male are principally the enlargement of the pincher 

 claws, and the different form and structure of the tail. In the normal 

 male the tail is narrower than in the female, and of the seven seg- 

 ments of which it is composed the 3rd, 4th and 5th are firmly united, 

 the joints between them being obliterated. In the infested male the 

 tail is considerably broader and the lines of division between seg- 

 ments 3, 4 and 5 are quite distinct. According to YvesDelages who 

 has made the most complete investigations of Sacculina, the crabs 

 are infested when from three to four mouths old, and from four to 

 twelve millimetres in diameter. Presumably at this time the special 

 modifications of the male abdomen have not appeared, and the normal 

 development of the testes being prevented by the presence of the Sac- 

 culiua, at first entirely internal, the external or secondary modifications 

 are also suppressed. It is possible, however, that even in the young 



