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Show 1896.] OF THE GENUS SERGESTES. 955 broad-thus conspicuously more narrow than in 8. mediterraneus, m.; and the outer margin beyond the spine but slightly concave. The branchiae (comp. the notes of S. I. Smith in Bull. Mus. Comp. Zool. vol. x. p. 96) do not seem to present any character fit for use. S. arctlcus, Kr., is smaller than the other species when the eyes obtain the black colour. Above I have mentioned that S. rubroguttatus, Wood-Mas., from the Indian Ocean is, in m y opinion, a valid species, as the exter. br. of urp. is described and figured (Ann. & Mag. Nat. Hist. ser. 6, vol. viii. p. 354) to be much more narrow than in S. arcticus, Kr., and without the spine on the outer margin. S. kroyerl, Bate, and S.prehensills, Bate, are unknown to m e ; they have the same branchial formula as S. arcticus, but a new investigation of both species is much needed ; perhaps S. rubroguttatus, Wood-Mas., is synonymous with S. krbgerl, Bate. Of S. arcticus, Kr., our museum possesses a series of all stages from the Acanthosoma (inch) to the mature form. One of these stages is S. rinkii, Kr. Kroyer's representation (p. 265, tab. ii. fig. 3, a-g) corresponds well with specimens of 8 m m . in length, rostrum not included, and is rather good; thus he describes and figures the eyes with their long stalks, the very characteristic antenn. ped., tbe shape of the squama, the dorsal spines on the abdomen, the long pleopods, the uropods with their exterior branch being very characteristic for the young Mastigopus, viz. 6-5 times longer than broad and the ciliated part of the exterior margin considerably longer than in the adult, finally the telson, which has a shape very different from that found in the adult-but the representation of the carapace is deficient (see later on) and misleading in one particular. Thus he describes the rostrum as being short, but it must already then have been broken off in one or two of his specimens; in reality it is about as long as the eye-stalks (without tbe eyes), and adorned at the basis with a dorsal spine almost as long as the diameter of the eye. The largest specimen of Acanthosoma, which, however, I shall not try to describe, is, the rostrum not included, 5-3 m m . in length, and with the rostrum (which reaches somewhat in advance of the eyes) c. 6*6 m m . long. Among the type specimens of Kroyer I find two specimens, which must be the stage immediately succeeding the Acanthosoma; one specimen is with the rostrum 6-9 mm. long, but from another locality I have seen a specimen in the same stage measuring even 8 m m . This stage differs considerably from that described by Kroyer, and therefore a short account of it shall be given. The rostrum is exceedingly long, reaching a little in advance of the eyes, on the distal part adorned with some short and fine setae, and at the basis originates a setaceous dorsal spine, which is adorned with some short and fine setae and is longer than tbe diameter of an eye. The supraocular spine is well developed, and the hepatic spine exceedingly long, considerably longer than the diameter of an eye; just in front of the gastro-hepatic groove is observed a short protuberance in the median line. The eyes |
