1915.] Fauna of the Chilka Lake : Mysidacea. 155 



Of the male specimens fifteen are adult and have the exopod of the third pleopods as 

 I have described it above. This may for convenience be known as form A. Two of 

 the males, while apparently adult (that is, they are quite as big as the other speci- 

 mens and are apparently, therefore, fully grown) have the exopod of the third pleopods 

 of a quite different form, which may be known as form B (fig. id, p. 157). The last 

 two joints are longer and not so stout as in form A and the bifid spine-like processes 

 are absent, being replaced by two simple spines. The single microscopically ridged 

 process is present as in form A, and the terminal curved spine is of the same propor- 

 tional length. In all other characters these two specimens conform to the type, and 

 it is to be noted that even in the exopod of the third pleopods, the same parts are 

 present on the last joint in both forms, viz., two spines and a single ridged process on 

 the inner lateral margin and a longer and a shorter spine at the apex. It is in the shape 

 of the last two joints and the character of the two lateral spines that the two forms 

 differ. Now the remaining seven males in this material are immature and the exopod 

 of the third pleopods is of a form which will ultimately result in the form B of male 

 pleopods, with further growth. It seems to me, therefore, that the two largest speci- 

 mens of form B cannot be quite adult, in spite of their size compared with the size 

 of form A and their mature look, and that the form B of male pleopods is a growth 

 stage in the formation of the form A type. It cannot, I take it, be a case of 

 " seasonal dimorphism " of the males (as for instance has been found by Wollebaek 

 for the males of Pandalus montagui, in which the shape of the endoped of the first 

 pleopods is of two forms, identified with the breeding and non-breeding seasons of the 

 species) because forms A and B were found mixed together in the one bottle and 

 therefore presumably captured together. It is possible that it is a case of definite 

 dimorphism in the males but, if so, I cannot understand why the exopod of the third 

 pleopods in the undoubtedly immature males should in all cases be of the form B type. 

 A fourth explanation is possibly open, that we have here a case of high and low 

 dimorphism among the males of this species, but the available data are insufficient 

 to decide the question. I incline to the opinion that forms A and B are the final 

 and penultimate stages in growth. It is unfortunate that the brush of setae on the 

 antennules, which in most other Mysidae is well developed in adult males, should be 

 feebly developed in Gastrosaccus so that this additional external mark of sexual 

 maturity is not here available as a guide. Moreover the separation of species becomes 

 more difficult because of the high systematic value hitherto set on the characters 

 displayed by the pleopods of the male. At the same time, the form B of G. muticus, 

 while resembling G. simulans more than form A in the shape of the exopod of the third 

 pleopods of the male, offers no possibility of confusion with the latter because, apart 

 from the differences in these appendages, the number of spines on the margins of the 

 telson affords an additional distinguishing character. 



Gastrosaccus simulans, sp. nov. 

 Locality. — Puri Beach, Orissa coast, washed up on shore, January, 1911, coll. 

 F. H. Gravely, three adult females, 7-8 mm., one adult male, 7-5 mm., one imma- 



