F. A. Potts 
51 
in definitely asserting a connection between their occurrence and 
parasitic influence, for bisexual gonads have to my knowledge never 
been met with in Decapod Crustacea under normal conditions 1 . But 
it thus appears that the curious condition in the hermit-crab is an 
incipient stage corresponding to the perfect hermaphroditism of the 
“ recovered ” spider crabs, and if the action of the parasite in absorbing 
surplus nutrition were withdrawn the young ova in the testis of the 
hermit crab would become large and pigmented like those in the 
spider crab 2 . 
These two cases have been described at some length as examples 
of extreme modification. In other Decapod Crustacea which are infected 
by the same parasite an effect is observable which is similar in kind 
but not in degree. The common shore crab of England ( Carcinus) 
is commonly afflicted (if affliction it be) by Sacculina. Here again 
the male undergoes modification while the reverse change never occurs 
in the female. The narrow abdomen of the male is often exchanged 
at the moult after infection for one much broader but never attaining 
the full female width. One may look in vain however for any reduction 
of the copulatory styles or for the appearance of the smallest rudiments 
of swimmerets. The closure of the genital apertures nearly always 
follows parasitic attack in spider crab and hermit crab; but they never 
become blocked up in shore crabs with Sacculina. Yet the external 
change is apparently greater than that produced in the reproductive 
glands. Dissection in every parasitised male showed vasa deferentia 
of the characteristic milky white colour due to countless masses of 
spermatophores all packed with spermatozoa. The testes though 
reduced, then, always remain in reproductive activity. The parasites 
which infect spider crab and shore crab are practically identical and 
presumably exert a very similar stimulus, yet the results are markedly 
different. It is obviously the host which offers a different reaction in 
the two cases. In another crab (Eripliia) examined by Smith there 
was infection both by Sacculina and by a parasitic Isopod crustacean. 
Here the nature of the parasite governs the result, and crabs with 
Sacculina alone never showed the least trace of modification, while 
changes closely similar to those described above occurred in those which 
harboured the Isopod. 
1 Caiman in the recently-appeared volume Crustacea of Ray Lankester’s Treatise on 
Zoology refers to the unpublished observations of Wolleback on normal hermaphroditism 
in certain deep-water Decapoda. 
2 Experiments, like those of Smith’s, planned to discover the fate of these ova in 
recovered hermit crabs were unfortunately inconclusive. 
4—2 
