1915-] Fauna of the Chilka Lake : Crustacea Decapoda. 293 



Oxyrhynch crabs ; but it does not seem probable that the two cases are really- 

 identical, though in both it is the development of the first legs that is concerned. 



According to Geoffrey Smith's investigations ' three types of males are to be 

 found in some species of Oxyrhyncha and he names these three types "low," 

 "middle" and "high." In both low and high males the chelae are swollen, there 

 being a marked difference between the two groups in the comparative size of the 

 limbs. The chelipedes of the middle form are, on the contrary, scarcely swollen at 

 all, resembling those of the females. The low males are smaller than the high and 

 the middle intermediate in size. During the breeding seasons the majority of the 

 males that are found belong to the low or high forms, while in the intervening seasons 

 middle males predominate. The low male in the course of its development to the 

 high form passes through a middle stage in which the sexual function is in abeyance. 



The investigations made by Hagen 2 and Faxon 3 on American crayfish referred 

 to Cambarus have brought to light the fact that in this genus there are two forms 

 of male, distinguished by the shape of the first abdominal appendages, and it has 

 been shown that these two forms represent breeding and non-breeding phases. An 

 almost precisely similar phenomenon has been noticed by Wollebaek * in one of the 

 Pandalidae, Pandalus montagui (= annulicornis) , though it apparently does not 

 occur in allied species of the genus. 



It is evident that these two last instances, although the organs concerned are 

 more directly connected with the sexual function, belong to the same category as 

 that of the Oxyrhynch crabs, in which the peculiarities are shown in the chelipedes. 

 Although phenomena of the kind seem to be rare in the Decapods, it is clear that 

 instances of a seasonal sexual dimorphism occur in at least three widely separated 

 groups of the order, viz. the Caridea, Nephropsidea and Oxyrhyncha. 



True dimorphism, i.e. the " definitive dimorphism " of Smith, is well known in 

 many insects ; but, if it ever exists, is of extremely rare occurrence among Deca- 

 poda. 6 Henderson and Matthai 8 have, indeed, brought forward facts which seem to 

 indicate that the Palaemonidae known as Pal.aemon scabric-ulus , P. dolichodactylus 

 and P. dubius are in reality true trimorphic forms of a single species, differing from 

 one another in the proportionate measurements of the large claws of the male. Fur- 

 ther evidence is, however, necessary before this view can be accepted as definitely 

 proved. 1 



It appears to be impossible to bring the case of Athanas polymorphus into line 

 with any of these instances. That the males show a definitive trimorphism is, I 



1 Smith, Mitth. zool. Stat. Neapel, XVII, p. 312 (1905). 



2 Hagen, III. Cat. Mus. Comp. Zool. II, pp. 20, 21 (1870). 



3 Faxon, Mem. Mus. Comp. Zool., X, 4, p. 12 (1885)0' 



4 Wollebaek, Bergens Museums Aarbog, 1912, No. 8, p. 64. 



6 See my paper in Rec. Ind. Mus., X, pp. 84-87 (19 14) for a criticism of the supposed dimorphism 

 in certain Hippolytidae and Palaemonidae. 



6 Henderson and Matthai, Rec. Ind. Mus., V, p. 280 (1910). 



1 A definitive dimorphism is of course well known in Crustacea other than Decapods. 



