74 The effect of the Parasites on their Hosts. 



Firstly, that a perfect external hermaphroditism can be associateci with 

 the entire absenceof a differentiated gonad. Secondly, that the male sex, 

 and probably the male sex alone, can be so radically modified in its sexual 

 nature as to assume a perfect external hermaphroditism. 



In the next section we describe the remarkable phenomena which accompany the re- 

 covery of these crabs from their parasitic affection. 



D. Tlie sexual condition of recovered crabs. 



1. In a state of nature. 



When the Sacculina has produced a certain number of broods and has finished its 

 terni of existence, it drops off, leaving a scar by which crabs that have been once infected can 

 be easily recognised. I have found a fair number of such recovered crabs in nature, and I 

 have artificially freed crabs of their parasites in the aquarium. The roots of the parasite 

 remain alive for some time, but if the crab does not die, as it usually does, the roots may 

 be resorbed to some extent after a few months. In three or four male crabs which had not 

 undergone much external modification the testes had recovered considerably and were functional, 

 but I have only observed one recovered female Avith mature ovaries. 



The most important discovery I have made relating to recovered crabs 

 is that under certain circumstances the crab regenerates a perfect hermaphro- 

 dite gland. I have found altogether 4 such crabs; they were ali perfect hermaphrodites 

 externally as well as internally, and one of them had formed oviducts and vasa deferentia, 

 while in the other three the ducts were absent. The gland was paired, the upper portion on 

 either side being a testis, the lower an ovary ; the testis in ali cases containing fully differen- 

 tiated spermatozoa, and the ovary large pigmented ova, which were nearly mature. Piate 7 

 fig. 20 is a low-power drawing of a section of part of the ovotestis, showing two ovarian and 

 one testicular tube. Piate 7 fig. 20 a is a section taken through the part of the gland where 

 the testicular passed into the ovarian tubes, and it is seen that in this region one and the 

 same tube is giving rise to ova and spermatozoa. 



Ali these curious crabs, except one, showed unmistakable signs of having been infected 

 with Sacculina, the characteristic chitinous ring of attachment being conspicuous on the ventral 

 side of the abdomen, and since no kind of hermaphroditism has ever been found in I. scorpio 

 apart from the effect on the external characters caused by Sacculina, we are quite safe in con- 

 cluding that the sole cause of the internai hermaphroditism of these 4 crabs was the fact that 

 their gonads had been eradicated by Sacculina and had subsequently regenerated. The gonads 

 must first have been practically eradicated, because these crabs belong to the category in 

 which the external characters are truly hermaphroditic (Piate 7 figs. 14 — 19), and it has been 

 proved that crabs of this category never have differentiated gonads, or at most the degenerate 

 remains of a testis (see Paragraph C). 



