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Memoirs of the Indian Museum. [Voi,. V, 



impossible to do more than make a few suggestions, some perhaps rather more prob- 

 able than others, to account for the existence of this curious phenomenon. 



It will be seen from the table given above that the largest and most fully 

 developed specimens, Forms II and III, were caught in March, while all belonging to 

 Form I are smaller and were obtained in September. The breeding season is appar- 

 ently in March, for it was only in this month that ovigerous females were found. 

 Judging by its size. Form I is probably the youngest and may represent a non- 

 breeding pha.se; but it would be very extraordinary if the three forms were merely 

 stages in the life -history of the species. In very young males, perhaps early post- 

 larval forms, it is probable the first limbs are both slender, resembling those of 

 the female and, if Forms I, II and III represent successive stages in growth, the 

 development is, as far as I am aware, without parallel. On this theory the young 

 male would, in the course of a few months, develop one enlarged limb, the pair 

 being greatly asymmetrical (Form I). Later, at a subsequent moult, the other limb 

 would be similarly enlarged, the pair thus becoming symmetrical (Form II), while 

 still later asymmetry would again be manifest in the form of the chelae, the spines 

 on the merus being increased in number in both limbs. According to this theory 

 the males could not strictly speaking be regarded as trimorphic, the case would 

 merely be one of a most unusual post-larval metamorphosis. 



Another and perhaps rather more plausible suggestion may be made. The 

 males may be strictly dimorphic, Forms II and III each representing the ultimate 

 development in the life of an individual, each being a fixed type which never alters 

 in the course of subsequent moults. On this theory Forms II and III would be devel- 

 oped simultaneously at the beginning of the breeding season from the non-breeding 

 phase represented by Form I. 



One more theory remains. The males may be strictly trimorphic, each of the 

 three forms representing a fixed and unalterable type, predetermined perhaps from 

 an early age. The facts available regarding the size and date of capture of the 

 specimens seem to indicate that this view has but little to recommend it. 



The three suggestions which have been made must, I think, exhaust all probable 

 explanations of the case, for it is impossible that the three forms merely represent 

 items in a series exhibiting normal variation. My suggestions may be summarised 

 thus : — 



Theory I. That the three forms of male represent merely so many stages in 



the life-history of this sex of the species. 

 Theory II. That the males are truly dimorphic. Forms II and III representing 



unalterable types developed simultaneously at the breeding season 



from the non-breeding Form I. 

 Theory III. That the males are truly trimorphic, each of the forms representing 



a type unalterable in the life of the individual. 



At first glance it seems possible to find an analogy between the three forms of 

 male in Athanas polymorphus and the three forms of the same sex known in certain 



