The most completely studied cases of parasitic castration are those of the Rhizocephalous Sacculina neglecta, parasitic on the spider-crab, Inachus mauritanicus,[[75]] and of Peltogaster curvatus on the Hermit-crab, Eupagurus excavatus, var. meticulosa.[[76]] The ordinary males of I. mauritanicus have the appearance shown in Fig. [72], A. The abdomen is small and bears a pair of copulatory styles, while the chelipedes are long and swollen. In the female (B) the abdomen is much larger and trough-shaped, and carries four pairs of ovigerous appendages; the chelae are small and narrow.
Fig. [72].—Illustrating the effect of parasitic Sacculina neglecta on Inachus mauritanicus, nat. size. A, Normal male; Inachus; B, normal female; C, male infested by Sacculina (final stage); D, abdomen of infested female; E, infested male in an early stage of its modification.
Now it is found that in about 70 per cent of males infected with Sacculina the body takes on to varying degrees the female characters, the abdomen becoming broad as in the female, with a tendency to develop the ovigerous appendages, while the chelae become reduced (Fig. [72], C). This assumption of the female characteristics by the male under the influence of the parasite may be so perfect that the abdomen and chelae become typically female in dimensions, while the abdomen develops not only the copulatory styles typical of the male, but also the four pairs of ovigerous appendages typical of the female. The parasitised females, on the other hand, though they may show a degenerate condition of the ovigerous appendages (Fig. [72], D), never develop a single positively male characteristic. On dissecting crabs of these various categories it is found that the generative organs are in varying conditions of degeneration and disintegration.
The most remarkable fact in this history is the subsequent behaviour of males which have assumed perfect female external characters, if the Sacculina drops off and the crabs recover from the disease. It is found that under these circumstances these males may regenerate from the remains of their gonads a perfect hermaphrodite gland, capable of producing mature ova and spermatozoa. The females appear quite incapable, on the other hand, of producing the male primary elements of sex on recovery, any more than they can produce the secondary. Exactly analogous facts have been observed in the case of the hermit-crabs parasitised by Peltogaster, but here the affected males produce small ova in their testes before the parasite is got rid of. Here, too, the females seem incapable of assuming male characters under the influence of the parasite.
To summarise shortly the conclusions to be deduced from these facts—certain animals react to the presence of parasites by altering their sexual condition. This alteration consists in the female sex in an arrest of reproductive activity, in the male sex in the arrest of reproductive activity coupled with the assumption of all the external characters proper to the female. But in these males it is not merely the external characters that have been altered; their capacity for subsequently developing hermaphrodite glands shows that their whole organisation has been converted towards the female state. That this alteration consists in a reorganisation of the metabolic activities of the body is clearly suggested, and in the succeeding paragraph we furnish some further evidence in support of this view.
Fig. [73].—Inachus mauritanicus, × 1. A, Low male; B, middle male; C, high male; the great chela of the right side is the only appendage represented.
Partial and Temporary Hermaphroditism. High and Low Dimorphism.
The reproductive phases of animals are frequently rhythmic, periods of growth alternating with periods of reproduction. This is well exemplified in the case of the ordinary males of Inachus mauritanicus, of some other Oxyrhynchous crabs, and of the Crayfish Cambarus.[[77]] During the breeding season the males of I. mauritanicus fall into three chief categories: Small males with swollen chelae (Fig. [73], A), middle-sized males with flattened chelae (B), and large males with enormously swollen chelae (C). On dissecting specimens of the first and third categories it is found that the testes occupy a large part of the thoracic cavity and are full of spermatozoa, while in the middle-sized males with female-like chelae the testes appear shrivelled and contain few spermatozoa. These non-breeding crabs are, in fact, undergoing a period of active growth and sexual suppression before attaining the final state of development exhibited by the large breeding males. This phenomenon is obviously parallel to the “high and low dimorphism”[[78]] so common in Lamellicorn beetles, where the males of many species are divided into two chief categories, viz. “low males” of small size in which the secondary sexual characters are poorly developed, and “high males” of large size in which these characters are proportionately much more highly developed than in the low males. The only difference between the two cases is that whereas in the beetles growth ceases on the attainment of maturity in the low degree, in the Crustacea the low male passes through a period of growth and sexual suppression to reach the high degree of development.