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Show 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 larvae and the adults is, as hinted above, the shape and especially the colour of the eyes : in tbe larvae 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 larvae, the eyes are yellowish (or whitish), and black pigment, when present, Is only found In the Interior and very remote from the cornea, ivhlle in the adults 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 tbe youngest Mastig opus-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 w e arrive at three fresh considerations : (1) the separation of the adult species from each other ; (2) the discrimination of the larvae, so that the different stages of the same Mastigopus may be referred to each other and separated from other larvae; and (3) the reference of any given Mastigopus to its species of Sergestes. In the literature of tbe 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 Krd 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.1, their size, and the shape of the basal one, the shape of the apical part of the squama, the length and structure of mxp.3 (whether the 4 proximal joints are similar to those in trl.3 or are obviously incrassated, the arming 1 In order to abridge the descriptions, I in the following pages make use of some abbreviations:-antenn. ped. = peduncle of the antennuke, mxp.3 = 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 ZOOL. Soc-1896, No. LXI. 61 |
