1896. ] OF THE GENUS SERGESTES, 945 
considered as the real Mastigopus, and even sometimes to determine 
the Acanthosoma, consequently to elucidate at least one-half of the 
total metamorphosis. As a rule the differences between the same 
older stage of any two species whatsoever are more conspicuous than 
the differences between the species of Sergestes to which they 
belong. During a long-continued study of a rich collection it will 
gradually be possible to arrive at complete certainty in the 
collocation of the series of stages of all species well represented, 
but in too numerous instances it is impossible to refer the forms 
to the representations of authors without examining their type- 
specimens, 
The characters which undergo very little or almost no change 
during the metamorphosis from the older Mustigopus to the adult, 
and for that reason offer good marks for identification, are the 
following :—(1) the structure of mxp.3, viz., whether they are 
scarcely longer than trl.* with the 4 proximal joints of the same 
aspect (the fourth joint flattened) as in trl.* and the two distal 
joints equally setaceous on both margins—or whether they are 
considerably or much ionger than trl.*, with the 4 proximal joints 
considerably thickened and much more robust than in trl.3, and 
the 2 distal joints almost or totally naked along the one margin, 
while at least the sixth joint is armed with several long and a 
number of shorter spines on the other margin (but only the 
presence of spines, not their number, can here be taken into 
consideration ; (2) the proportion between the naked and the hairy 
part of the external margin of ext. br. of urp.; (8) the relative 
length of the 3 joints (especially the first and the third) of the 
antenn. ped.; (4) the number of subjoints in the sixth joint 
of mxp.*—Other characters of more secondary value will be pointed 
out in dealing with the species. 
The character derived from the length of mxp.°, and especially 
from the aspect of their 4 proximal joints in contradistinction to 
the legs and especially to trl.*, can also be used in every Mastigopus- 
stage; it will even almost always be possible to refer a larva with 
mxp.° broken off to one of the two groups by comparing the basal 
joint, which always persists, with the basal joints of the 3 
following pairs of trunk-legs.— The character from the uropods is 
in most cases more or less subject to alteration during the 
development, and as a general rule it may be stated, that when only 
3-4 of the exterior margin is hairy in the adult, then this part 
approximates more and more towards occupying 4 of the margin 
according to the youth of the specimens ; but when the margin is 
hairy in the total or in c. $ of its length in the adult, then the 
hairy part is a little shorter in the younger, and still somewhat 
shorter in the youngest Mastigopus—The character from the 
length of the joints in antenn. ped. also alters in the younger 
stages, with the result that the first joint is proportionally longer 
(and aistally much narrower) in the younger than in the older 
Mastigopus-stages.—Several instances proving these rules will be 
found in the following descriptions of the species. 
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