274 THE CRUSTACEA 



modified, as described below) alone persist in the male, while in 

 the female the second to the fifth pairs are (with rare exceptions) 

 developed as egg-carrying appendages, with short protopodite and 

 long and slender rami ; the first pair are absent in the female ex- 

 cept in the Dromiacea. In Callianidea (Thalassinidea) the rami of 

 the pleopods are fringed with long filaments, apparently branchial in 

 function ; this isolated case forms a curious parallel to the develop- 

 ment of branchial filaments on the pleopods in the Stomatopoda 

 and in Bathynomus among the Isopoda. 



Sexual modifications are commonly presented by the pleopods, 

 most constantly by those of the first and second pairs, Avhich in the 

 male assume a copulatory function. In the case of the first pair 

 the difference may be slight, as in most Caridea, where the endo- 

 podite is reduced to a small leaflet, differing more or less in shape 

 in the two sexes, and in the male armed with a group of coupling- 

 hooks. In the Penaeidea the first pair of the female have the 

 endopodite small or wanting ; in those of the male it is represented 

 by a membranous plate, often of large size and complicated struc- 

 ture, attached to the inner side of the peduncle, and bearing (as in 

 the Caridea) a group of coupling-hooks which interlock with those 

 of the other side. To this apparatus the name of petasma has been 

 given by Spence Bate. In the Reptantia the appendages of this 

 pair are never biramous. In the female sex they are greatly reduced 

 in size or altogether absent. Occasionally they are absent in both 

 sexes (Parastacidae, Scyllaridea, some Paguridea, and Hippidea), 

 but more commonly they are developed in the male into copulatory 

 appendages, usually styliform, with a spoon -shaped or tubular 

 terminal part. In some Thalassinidea (Upogebia), by a rare excep- 

 tion, these appendages are present (uniramous) in the female but 

 absent in the male sex. 



The second pair in the female sex are almost always similar to 

 those which follow. In the male sex, however, this is rarely the 

 case (some Scyllaridea, Parastacidae, Upogebia). As a rule, they 

 are modified by the development of an accessory process, the 

 appendix masculina (Boas), from the inner edge of the endopodite. 

 This appendix is small in the Penaeidea and Caridea (in which latter 

 it may coexist with the appendix interna), but in the other groups 

 it increases in importance, the terminal part of the endopodite 

 diminishing, as does also the exopodite, until in the Brachyura 

 (and some Anomura) there remains only a styliform appendage of 

 two segments, the proximal representing the protopodite and the 

 distal the endopodite together with its appendix masculina. 



The wopods retain in the Macrurous groups the general 

 characters of the caridoid type, having a short protopodite and 

 broad lamellar rami, forming with the telson a tail-fan. As a rule 

 the exopodite is more or less distinctly divided by a transverse 



