A STUDY IN MORPHOLOGY. 101 



V. THE METAMORPHOSIS OF ACETES. 



While I was studying the development of Lncifcr, I found during the month of 

 September a few specimens of the very similar larva which is shown from above in 

 Plate 9, fig. 79, and from the side in fig. 78. 



Several specimens were placed by themselves in tumblers of sea-water, where they 

 passed through the stages shown in Plate 11, figs. 84, 85, and 90. Only one of my 

 specimens reached this last stage, and as this one moulted on the last day of the 

 season I was not able to trace it any further, and as I collected no adult specimens of 

 the same kind, its precise systematic position must at present remain in some 

 uncertainty. The close similarity which I shall point out between its larval stages 

 and those of Lucifer and Sergestes renders it very probable that it is a Sergestid, and 

 the analogy of these forms also indicates that the larva shown in fig. 90 has in all 

 probability nearly or quite attained to the mature form. This larva differs from the 

 other two forms in the possession of small claws at the tips of the last three pairs of 

 pereiopods, and as this is characteristic of MILNE-EDWARDS' genus Acetcs, and only 

 three genera of Sergestidse Lucifer, Acetcs, and Sergestes have been described, I 

 think we may conclude that we have to do with the development of an American 

 species of this genus. At any rate, whatever the systematic position of the adult may 

 be, the fact that the Protozoea is in most respects intermediate between the simple 

 Protozoea of Lucifer and the extremely modified Protozoea of Sergestes, gives this form 

 so much interest that it seemed best, for the sake of comparison, to embody all that 

 I was able to learn about its metamorphosis in the present paper. 



At the earliest stage which was observed, the larva (figs. 77, 78, and 79) is a "Zo'e'a" 

 1000 inch long, and a comparison of fig. 79 with fig. 44, or of fig. 77 with fig. 43, will 

 show that it is essentially like the last Zoca of Lucifer, although the minor differences 

 are both numerous and conspicuous. 



The number of somites and appendages is alike in both forms, and the appendages 

 are alike in most respects, although each one of them shows distinctive characteristics 

 of its own. 



The carapace (fig. 79) makes about one-half the length of the body, and it is much 

 more flattened than it is in Lucifer. It has a rostrum (R) and a median dorsal spine, 

 but the postero-lateral spines (sp.) point outwards and backwards, instead of directly 

 backwards, and there are a pair of anterior spines as long as the rostrum, projecting 

 over the eyes. The two large pigment-spots which give such a characteristic appear- 

 ance to the carapace of Lucifer are entirely absent, and the thoracic segments and 

 appendages are covered by its posterior edge. 



The eyes are mounted upon distinct stalks, while they are sessile in Lucifer at the 

 same stage. 



The abdominal somites are short and wide, and coloured by bright-red pigment- 

 spots, and their lower edges are produced into strong projecting spines. 



