1896. ] OF THE GENUS SERGESTES. 955 
broad—thus conspicuously more narrow than in S. mediterraneus, 
m.; and the outer margin beyond the spine but slightly concave. 
The branchiz (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. arcticus, 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 my 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 8. arcticus, Kr., 
and without the spine on the outer margin. S. kréyeri, Bate, and 
S. prehensilis, Bate, are unknown to me; 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. kroyeri, Bate. 
Of S. arcticus, Kr., our museum possesses a series of all stages from 
the Acanthosoma (incl.) to the mature form. One of these stages 
is S. rink, Kr. Kréyer’s representation (p. 265, tab. ii. fic. 3, 
a—q) corresponds well with specimens of 8 mm. 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., 
the 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 par- 
ticular. 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 the 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, 5:2 mm. in length, 
and with the rostrum (which reaches somewhat in advance of 
the eyes) c. 6-6 mm. long. Among the type specimens of Kréyer 
I find two specimens, which must be the stage immediately suc- 
ceeding 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 8mm. ‘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 sete, and at the basis originates a setaceous dorsal spine, 
which is adorned with some short and fine sete and is longer than 
the 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 
