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Part III. — Twenty-first Annual Report 



The number of aesthetascs was found to be — four in the proximal 

 group, four in the next, three in the distal^ and beyond that group a 

 single aesthetasc arose from the middle of the end joint. 



The exopodite bears a long smooth terminal hair. 



A few minute "needle-point" cilia were found on the second joint. 



Antenna, A., PI. vii. 



I. (Fig. 16). The antenna of the I. Zoea consists of a single basal 

 joint which is produced distally into a long spine, serrated on both sides 

 (Fig. 1, h). At the distal end of the joint there articulates a branch 

 consisting of a long narrow joint which bears two smooth spines, one 

 very long, the other short. The distal end of the long joint is serrated 

 (Fig. 1, «), and the short spine rises from a projecting corner of the 

 same. This branch is the exopodite, and it is the homologue of the 

 scale of the antenna of Crangon vulgaris. It is present in the Zoea, 

 but absent in the adult. 



The serrated process is the endopodite, and from it is developed the 

 flagellum, i.e. the antenna of the adult. On the proximal part of the 

 process there is a swelling {sw, Fig. 16), which is the rudiment of the 

 flagellum. During the succeeding Zoea stages it grows in size along- 

 side but quite independently of the serrated process. In this respect 

 Carcinus differs from Crangon. In the latter the serrated process 

 develops into the flagellum. The future whip first appears in the wide 

 proximal part of the process, and as it grows it absorbs the serrated 

 portion till the latter becomes merely a tuft of hairs on the end of the 

 former. In Carciiius, however, in the IV. or last Zoea stage the 

 serrated process is as large' and important proportionally as in the I. 

 Zoea, 



The serrated process in both forms is a character of the Zoea alone. 



During the succeeding Zoea stages, development in the antenna takes 

 place only in the bud of the flagellum, the other parts remain unchanged 

 in form, though they increase in size. 



II. (Fig. 17). The rudimentary flagellum is now prominent as a 

 conical protuberance. 



III. (Figs. 4, 5). The flagellum is now half the length of the serrated 

 process. 



lY. (Fig. 6). The flagellum is now almost as long as the serrated 

 process. The exopodite has diminished in l elative size. 



Megalops (Fig. 11). The antenna now has its adult form. It 

 consists of nine joints, the first three are wider than those that follow. 

 The latter, with one exception, viz. the fourth joint of the antenna, are 

 long narrow tapering joints. The antenna is furnished with two 

 prominent long smooth stiff hairs. They arise from the distal end of 

 the seventh joint. With them are associated two short hairs. The 

 terminal joint bears three stiff smooth slender hairs, and round the 

 distal edge of the fifth joint there is a ring of four short hairs. On the 

 third joint there is one short hair, and on the first joint there are three 

 similar hairs. In another Megalops the antenna had only eight joints, 

 the fifth and sixth joints of the form drawn being fused together. The 

 first joint of the antenna appears to be the protopodite, and is probably 

 the homologue of the basal joint of the antenna of the Zoea. 



1. (Figs. 7, 21, and 23). In the first young stage there is little change 

 from the condition in the Megalops. Considerable variation was found 

 in the antennae of different individuals. In one (Fig. 7) there were 

 ten joints ; the arrangement of hairs was practically the same, except 

 that a joint seemed to have been added to the first three joints, so that 



