A STUDY IN MORPHOLOGY. 89 



The first pereiopod (fig. 01, Pr. 1) is four-jointed, and shorter than the last 

 maxilliped. 



The second and third pereiopods (Pr. 2 and Pr. 3) are nearly equal, and twice as 

 long as the first ; they are four-jointed, have a double row of small hairs along the 

 anterior edge, and the last ends in a small curved hairy claw. 



They exhibit no trace of gills or of endopodites, and there is no stump to indicate the 

 position of the fourth pereiopod, which disappeared at the end of the Schizopod period. 



The first abdominal appendage of immature specimens or of mature females 

 (Plate 9, fig. 74, PL 1) is made up of a thick basal portion, which is unjomted 

 in young specimens but two-jointed in mature ones, and a pointed annulated terminal 

 portion which is fringed with swimming hairs. In the nearly grown but immature 

 male (fig. 76) there is a little bud or projection (a) near the base of the anterior 

 surface of the long basal joint. In the sexually mature male (fig. 75) this bud has 

 become the clasping organ which has been described by MILNE-EDWARDS, DANA, 

 SEMPER, DOHRN, GLAUS, FAXON, and others ; and another smaller process or tooth 

 has appeared upon the distal one of the two joints into which the base of the 

 limb has now divided. 



The second, third, fourth, and fifth pleopods consist, in the young of both sexes, 

 and in the mature females, of a long unjointed basal portion and two hairy terminal 

 branches. In the adult male the second pleopod has a third and smaller terminal 

 branch, as GLAUS has pointed out (Zeit. f. Wiss. Zool., xiii., 434). 



The first, second, third, fourth, and fifth abdominal somites end below in short spines, 

 and they are all about equal in length, except the fifth which is nearly twice as long as 

 any of the others. It has a median dorsal spine on its posterior edge, and the very 

 young specimens also have a pair of postero -lateral spines, as shown in Plate 7, 

 fig. GO. In older specimens this pair of spines disappears, as shown in Plate 9, fig. 72, 

 and in the adult female the somite undergoes no further change. When the male 

 reaches sexual maturity, however, the lower edge of the somite becomes produced, as 

 described by DANA, on each side into the hooks shown in fig. 73. In our species 

 the smaller one of these hooks is near the middle of the somite, and the larger 

 one about halfway between it and the posterior edge. 



As shown in figures 72 and 73, the telson of an adult specimen is only about half 

 as long as the swimmerets. The tip of the telson of an adult female is shown from 

 above in fig. 71. 



In the male the telson becomes somewhat bent (fig. 73, T) as maturity is reached, 

 and a rounded anal papilla becomes developed in its lower surface, while the telson of 

 the adult female remains like that of immature specimens of both sexes. 



The exopodite of mature specimens usually has about twenty hairs, and the endopodite 

 sixteen. The exopodite is longer and wider than the endopodite, and it is alike in 

 both sexes until maturity is reached, when it becomes somewhat modified in the male, 



MDCCOLXXXII. N 



