1896.] OF THE GENUS SERGESTES. 943 
legs are at least shorter, more slender, and with fewer hairs than in 
the adults. But the best distinction between the larve and the 
adults is, as hinted above, the shape and especially the colour of 
the eyes: in the larve the eye-stalks are almost always long, the 
eyes are rather large, or even very large, and have an oblique and 
more or less fungiform shape; while in the adults the eye-stalks are 
rather short, and the eyes smaller, more regularly globular, and 
sometimes but slightly thicker than the distal end of the stalk ; in 
all larve the eyes are yellowish (or whitish), and black pigment, when 
present, is only found in the interior and very remote from the cornea, 
while in the adulis the eyes are totally black. But it must be 
emphasized that even when the black eyes are acquired and all other 
larval characters have been lost, the animals are still immature, as 
the petasma is developed somewhat later, and the petasma itself 
does not become completely developed at once to its final shape. 
For the rest, more or less conspicuous alterations in all parts of 
the body and the limbs take place during the development from 
the youngest Mastigopus-stage to the adult Sergestes, but it is 
impossible to give a full elucidation without numerous figures. 
Besides, the species show considerable differences in development: | 
thus, for instance, the dorsal abdominal spines are in some species 
lost when the Mastigopus is not half-grown, while in other species 
they are preserved till the Mastigopus is almost full-grown and the 
colour of the eyes alters, &c. Therefore I do not attempt to 
give a general picture of the metamorphosis, but I will refer the 
reader to the following more special, but short treatment of the 
species. 
Next we arrive at three fresh considerations: (1) the separation 
of the adult species from each other ; (2) the discrimination of the 
larvee, so that the different stages of the same Mastigopus may be 
referred to each other and separated from other larve; and 
(3) the reference of any given Mastigopus to its species of Sergestes. 
In the literature of the subject numerous characters have been 
used, but some of them are only applicable to the adults, others to 
the larval forms, and several good characters proposed by Kré er 
and S. I. Smith have been overlooked, or at least not used with 
sufficient accuracy, by most authors. The whole question of the 
characters must be re-examined. 
For the characterization of the adult species must be used 
differences in the following structures :—the shape of the rostrum, 
absence or presence of supra-ocular spine, hepatic spine,and gastro- 
hepatic groove on the carapace, shape and size of the eyes, the 
relative length of the 3 joints of the antenn. ped.’, their size, and the 
shape of the basal one, the shape of the apical part of the squama, 
the length and structure of mxp.° (whether the 4 proximal joints 
are similar to those in trl.° or are obviously incrassated, the arming 
1. Jn order to abridge the descriptions, I in the following pages make use of 
some abbreviations :—antenn. ped.=peduncle of the antennule, mxp.?=the 
third pair of maxillipeds, trl.!-trl.°=the first to the fifth pair of trunk-legs, 
ext. br. of urp.=external branch of the uropods, 
Proc. Zoou. Soo.—1896, No. LXI. 61 
