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Show 1903.J METAMORPHOSES OF DECAPOD CRTJSTACEAXS. 27 II. CRANGON TRISPINOSUS Hailstone* Length of the first larva T8 to 2*0 mm.; average for twenty-five specimens being T9 m m . The body is light greenish yellow in colour, with a conspicuous branching chromatophore placed dorsally in the middle of the thorax. In general form the zoa?a resembles that of C. nanus, the body being comparatively thickset and tapering gradually backwards. The lateral edges of the carapace are deeply arched below and perfectly smooth except for a blunt spine-like prominence below the base of the second antenna (PI. YI. fig. 7). Anteriorly it is prolonged into a short pointed rostrum barely half the length of the peduncle of the first antenna. The carapace is not folded off over the eyes, so that the latter appear to be a part of it. The same pair of small outgrowths in front of the eyes as were described in C. fasciatus also occur in C. trispinosus (PI. YI. fig. 8). J^o trace of them can be detected in later stages. The abdominal segments have their posterior dorsal edges usually evenly rounded, without spines, but the fifth segment sometimes has a pair of short dorso-lateral spines, and traces of them can often be detected. The tail-plate is of the usual form, the spines being sharply pointed, and not blunt as in C. fasciatus (PI. YI. fig. 8). In the form of the appendages C. trispinosus differs so little from C. nanus, which has been fully described by Sars, that it is not necessary to describe them in full. The first antenna? of C. nanus, however, differ from those of C. trispinosits in that the inner flagellum is armed with a few minute spines in addition to the long cilia which are common to both (PI. YI. figs. 9 & 10). The second antenna? in the two species agree in all respects, but both species differ from C. echinulatus, which is in other respects closely similar, in the much narrower form of the scale. In the later stages of the larva the body becomes more compact and thick-set, having an appearance very characteristic of this species and of C. nanus, from which, however, it is easily distinguishable by the absence of spines from the abdominal segments and from the lower edges of the carapace. The rostrum is now very broad at the base, contracting suddenly near its end and continuing as a short, sharp spine. As far as the scanty material at m y disposal for the study of the later stages goes, the spines occasionally developed on the fifth abdominal segment of the first larva are lost with the first moult. The gills appear in the fourth stage of development. At this stage five gills can be seen placed, as it appears to me, in the position of pleurobranchs-that is to say, they are well above the apparent attachment of the legs (PI. YI. fig. 12). As the legs * Glaus has figured (1884, taf. vii.) a Crangon larva which almost certainly belongs to this species. The absence of abdominal spines, the form of the rostrum, and the presence of an exopodite on the second pereiopod identify the larva with C. trispinosus. |
