102 CRUSTACEA 



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 Saceidina drops off arid 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 \vhole 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. 



Partial and Temporary Hermaphroditism. High and 

 Low Dimorphism. 



The reproductive phases of animals are frequently rhythmic, 

 periods of growth alternating with periods of reproduction. 



