| OCR Text |
Show 1896.] OF THE GEJJUS 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 tbe 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 Mastigopus 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.3 with the 4 proximal joints of the same aspect (the fourth joint flattened) as in trl.3 and the two distal joiuts equally setaceous on both margins-or whether they are considerably or much longer than trl.3, 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.; (3) the relative length of the 3 joints (especially tbe first and the third) of the antenn. ped.; (4) the number of subjoints in the sixth joint of mxp.3-Other characters of more secondary value will be pointed out in dealing with the species. The character derived from the length of mxp.3, and especially from the aspect of their 4 proximal joints in contradistinction to the legs and especially to trl.3, can also be used In every Mastigopus-stage; it will even almost always be possible to refer a larva with mxp.3 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 m a y be stated, that when only \-\ of the exterior margin is hairy in the adult, then this part approximates more and more towards occupying \ of the margin according to the youth of the specimens ; but when the margin is hairy in the total or in c. f 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 distally much narrower) in the younger than in the older Mastlgopus-stages.-Several instances proving these rules will be found in the following descriptions of tbe species. |
