MUSEUM OP COMPARATIVE Z05L0GY. 315 



the next moult the larva assumes the form represented on Plate II. 

 Fig. 10. Considerable structural advance appears in the antenna), in 

 the acquisition of another pair of swimming-feet, and the rudiments of 

 all the abdominal appendages. 



The carapace now has two spines pointing forward in the median dor- 

 sal line. The inferior side of each of these is armed with minute, re- 

 curved teeth. The successive unfolding of the swimming-feet from 

 before backwards has been accompanied by a lengthening of the cara- 

 pace in a posterior direction, so that when the last pair has appeared 

 the bases of the whole line of limbs are covered by the branchiostegite. 



The rudimentary inner flagellum of the first pair of antenna) (PI. 11. 

 Fig. 11, ri) has increased a little in size, but is still composed of one 

 segment. The inner branch of the second antenna (PI. II. Fig. 12, r i) 

 has gained in length in relation to the outer branch, and is now divided 

 into three segments, of which the first and second are short and thick, 

 the third long and style-shaped. 



The sixth pair of biramous natatory appendages (the third pair of 

 Decapodal legs) are now present, and the first sign of the formation of 

 choice on the two antecedent pairs appears in the slight production of 

 the antero-internal angle of the penultimate segment of their inner 

 branches. As will be seen further on, this angle becomes produced 

 more and more until at last it forms a thumb opposable to the dactylus. 

 The first two pairs of swimming-feet are now quite small compared with 

 the following pairs, and begin to assume the proper function of max- 

 illipeds. Behind the sixth pair of swimming feet are the rudiments of 

 the following two pairs of thoracic appendages (PI. III. Fig. 1, XII,XII/; 

 Figs. 2, 3). The seventh pair (Fig. 2) are double; the eighth pair, or 

 hindmost thoracic appendages (Fig. 3), simple, sac-like struct ui'es. They 

 are bent forward underneath the thr)rax, and concealed by the appen- 

 dages in front of them. 



The appendages of the abdominal segments from the first to tlie fifth 

 inclusive arc now for the first time seen (PI. II. Fig. 10, XIV - XV111). 

 They are simple buds from the lower face of the abdomen, and in a side 

 view they are almost concealed by the pleurae. The appendages of the 

 sixth segment of the abdomen (PI. II. Fig. 15) have now acquired 

 a shape nearly like those of the adult prawn. The inner plate (r i) is 

 almost as long as the outer (?' e), and both are clearly defined from one 

 another and from the basal segment (protopodite). Both plates are pro- 

 vided with long and delicately fringed seta) arranged as represented in 

 the figure. 



