270 Memoirs of the Indian Museum. [Vol. V, 



very short, is to be seen on the carpus and palm, in the other specimens it is 

 invisible. 



As regards the proportions of the segments in specimens of different sizes, it will 

 be seen from the averages of the percentage figures that the most noticeable change 

 is that the ischium becomes proportionately shorter with increased size, i.e., it grows 

 much more slowly than the other segments. Henderson and Matthai have found that 

 this takes place in several Palaemons and it probably occurs in the males of most 

 species of the genus. In the case of P. ruais the disproportionate growth of this seg- 

 ment is counterbalanced by a considerable increase in the length of the palm and by 

 a less considerable increase in the carpus and dactylus. The merus in its relative 

 length remains practically constant during growth. 



Judging from the collection made in the Chilka Lake, the greater part of the 

 change in the proportionate lengths of the segments takes place suddenly. In males 

 in which the chelipedes are decidedly shorter than the total length (specimens up to 

 about 70 mm.) the proportions are similar to those of females. In larger individuals, 

 in which the larger chelipede is equal to, or less than one and a half times the total 

 length, notable differences are found; but the ischium is still proportionately larger 

 (125 %) than in the largest examples — those in which the larger chelipede exceeds one 

 and a half times the total length. From these facts it seems legitimate to infer that 

 the change from the female type of limb to that characteristic of the fully grown 

 male is, or may be, attained in two months. 



A striking feature of the series of males from which the measurements given on 

 p. 269 are derived, is that the appendix masculina is fully developed in all specimens 

 except no. 22, in which it is rudimentary. As far as I am aware no precise observa- 

 tions have been made on the age at which this stylet becomes evident; but, from its 

 intimate association with the sexual process, one would infer that it made its appear- 

 ance only when the testes became functionally active. That it should be perfectly 

 developed in specimens less than one quarter the maximum length of the species is 

 most remarkable. Henderson and Matthai have already shown that a precocious 

 sexual development may occur in at least some Palaemonidae, and it is probable that 

 P. rudis affords an instance of the same phenomenon. 



Three small males, nos. 23-25 of the series on p. 269, differ noticeably from any 

 others in the collection in the great relative length of the dactylus. They differ in 

 no other way from typical P. rudis of similar size, and I am inclined to regard them 

 as abnormalites ; it will be noticed that in normal specimens the dactylus is the most 

 variable of all the segments of the chelipedes in its proportional length. It is 

 possible that the great length of the fingers in these examples may indicate something 

 more than an abnormality and that individuals with this character may be aggre- 

 gated in certain localities to form a definite race ; but at present we have no evidence 

 that this is so. 



The measurements of a series of females are shown on p. 271 ; of these all except 

 the smallest (no. 11) bear eggs. The segments in their proportional lengths bear a 

 close resemblance to those of young males and show but little change during growth; 



