MODIFICATIONS OF STRUCTURE IN DECAPOD CRUSTACEA. 219 



the ancestors of Calappa had a similarly elongated orifice to 

 the exostegal canal, and that the serrations of the antero- 

 lateral margins in the modern species of Calappa are the last 

 relics of the marginal spines which covered the aflFerent 

 orifices in their ancestors. In C. granulata they have com- 

 pletely disappeared. In support of this contention I may 

 point out that the antero-lateral border of the carapace, which 

 in C. gallus is merely '^crenulate" in the adult, is "sharply 

 serrate" in the young (Alcock, 1896, p. 147). 



The denticulations of the clypeiform expansions are also 

 known to present a similar process of alteration from youth to 

 maturity (Alcock, 1. c. ; Henderson, 1893). No adequate ex- 

 planation, however, of the function of the clypeiform expan- 

 sions has been yet put forward. In view of Henderson's re- 

 marks concerning the prevalence of protective tints among the 

 arenicolous crabs of the Madras shores (1. c, 1893), the further 

 suggestion may be hazarded that the flattening and expansion 

 of the carapace in the shallow- water species of Calappa may 

 possibly indicate a process of selective assimilation towards the 

 appearance of empty bivalve shells. I am inclined, however, 

 to think that the explanation when found will probably in- 

 volve something more than mere protective resemblance. 



4. Upon the hairiness of the pterygostomial regions I can, 

 1 think, throw some positive light. Just as the marginal 

 spines serve as a sieve for fragments of sand and shell, so the 

 pterygostomial hairs serve as a sieve for the finer particles of 

 mud. In Calappa granulata the pterygostomial region 

 forms a triangular area, bounded on all three sides by a dense 

 row of fine hairs — a submarginal series beneath the antero- 

 lateral margin of the carapace, an internal series along the 

 exopodite of the third maxilliped, and a posterior transverse 

 series along the interior edge of the raeropodite (arm) of the 

 cheliped. The submarginal series is supplemented by a 

 carpet of fine hairs on the outer side of the branchiostegite, 

 the afferent channel being alone free. In my specimens these 

 hairs are full of mud, indicating that they have efficiently 

 discharged their sieve function during life, 



