50 
Parasitism 
particular histological change; but in the hermit crab it is curious 
to note the presence of large cells with large nucleus and abundant 
protoplasm in sections of the testis. These instantly suggest ova in 
their appearance and call to mind the instances of the occurrence of 
such cytological elements as a normal experience in the testes of many 
animals. In sand-hoppers ( Orchestia ) to quote a well-known case (and 
there are many others in the Crustacea) spermatozoa are produced in 
the anterior part of the young testis while posteriorly the whole space 
is occupied by two or three large ova (vide Boulenger, 1908). 
The particular interest of the phenomenon in this case is its 
association with a definite cause, that is, parasitism. We are also able 
to come to some conclusion as to the degree in which such a condition 
can be called true hermaphroditism. Some striking evidence is offered 
by spider crabs which were once infected by Sacculina but which have 
outlived their parasite and recovered from its influence. Such crabs 
occur in nature in fair frequency and the only reminder of their former 
condition is the chitinous ring on the abdomen which surrounded the 
peduncle of the parasite. After the death of the external part of the 
Sacculina the root system may continue to exist in the host and it 
is only when this has disintegrated and been absorbed that regenera¬ 
tion of the gonads becomes rapid, for the still living roots repress the 
development of the sexual organs as effectually as the living parasite. 
A few crabs however were found in which the gonads had again 
attained full size and maturity. One was a female with a well-de¬ 
veloped ovary and four were males only slightly modified externally, 
with glands producing large quantities of spermatozoa. The remaining 
four cases were remarkable for the crabs showed with a complete external 
hermaphroditism the corresponding gonads. In all four animals the 
reproductive gland consisted of a male part with ripe spermatozoa, and 
a female division with large pigmented ova. The ducts were usually 
absent, but one individual possessed both vasa deferentia and oviducts. 
The sequel to these observations is given by the experimental evidence 
which Smith then obtained. It was attempted to destroy the parasite 
by removing the external part and the crabs so freed were kept under 
comfortable conditions for several months and the few survivors then 
killed. Regeneration had obviously occurred to a considerable extent, 
but the gonads were nearly always unisexual. In one individual alone, 
which was externally a hermaphrodite there was a gonad similar to 
those just described. In spite of the comparatively small number of 
cases with fully formed hermaphrodite glands we are not going too far 
