| OCR Text |
Show 1896.] OF THE GENUS SERGESTES. 965 S. penerlnkl, Bate.-The specimen represented by Bate (p. 418, pl. lxxvi. fig. 3) is rather young, 8 m m . long. I have examined a number of specimens, between 6-4 and 14 m m . in length, partly from the Plankton expedition and partly from our museum. Specimens from c. 7"5 m m . to 14 m m . in length are easily distinguished from those of the same length of S. vlgilax, Stimps., by having the third, fourth, and fifth abdominal segments-in the younger specimens also the sixth segment-dorsally armed with spines, which in larger specimens are shorter than in the stage figured by Bate, but yet well developed ; the spine on the third segment is almost perpendicular. Iu older specimens the eye-stalks are somewhat shorter than iu S. vlgilax, Stimps., but yet long. In the older stages the rostrum is much shorter than in Bate's figure, but yet longer than in S. vlgilax, and its distal part is slender and directed horizontally forwards; in tbe younger stages it is towards ^ or more of the length of the eye-stalks and recalls somewhat that in 8. Incertus, m. (see below), but in specimens that have attained the length of 8 m m . it is destitute of a dorsal spine. In specimens c. 6-4 m m . long the rostrum is about half as long as the eye-stalks, with a very short dorsal spine a little way from its base. In the antenn. ped. the third joint is scarcely longer than the first. (In the young specimens the anterior abdominal segments are ventrally armed as described by Bate.) On the ext. br. of urp. the length of the ciliated part varies, as in S. vlgilax, in accordance with the length of the specimens, occupying from | to -| of the exterior margin; a tooth is present in the younger, not in the old specimens. The adult form is described above, bearing the same name. S. Incertus, m., is the Mastigopus of the adult described above. I have seen rather numerous specimens from 6-2 to 13 m m . in length. They are more slender than the corresponding stages of S. penerlnkl, Bate, which they closely agree wdth in the antenn. ped., the length of the eye-stalks, and the dorsal armature on the abdominal segments. But they are easily distinguished from this species by the ext. br. of urp., on which the ciliated part in all specimens occupies scarcely f of the exterior margin, and the spine is rather long. Moreover, the rostrum, which • in proportion to the length of the animal is from more than the half to scarcely ^ of the length of the eve-stalks, is rather characteristic: seen from the side the basal part is rather short and directed obliquely forwards and upwards, and then it suddenly becomes produced into a slender and distally very fine spine much longer than the basal part and quite horizontal; at the distal end of the basal part the upper margin Is armed with a fine spine, which is very short in the older specimens, and just beyond which the margin is somewhat concave in outline. In the young specimens the first two abdominal segments are ventrally in tbe median line armed with a lobe produced to a spine, and in these and even in specimens c. 10 m m. long the posterior margin of the carapace is armed with an erect spine. |
