1896.] OF THE GENUS SERGESTES. 965 
S. penerinki, Bate.—The specimen represented by Bate (p. 418, 
pl. Ixxvi. fig. 3) is rather young, 8 mm. long. I have examined a 
number of specimens, between 6:4 and 14 mm. in length, partly 
from the Plankton expedition and partly from our museum. 
Specimens from ce. 7°5 mm. to 14 mm. in length are easily distin- 
guished from those of the same length of S. vigilax, 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. In older specimens the eye- 
stalks are somewhat shorter than in S. vigelaw, Stimps., but yet 
long. In the older stages the rostrum is much shorter than in 
Bate’s figure, but yet longer than in S. vigilaw, and its distal part 
is slender and directed horizontally forwards; in the younger 
stages it is towards 3 or more of the Jength of the eye-stalks and 
recalls somewhat that in S. incertus, m. (see below), but in 
specimens that have attained the length of 8 mm. it is destitute 
ot a dorsal spine. In specimens ec. 6-4 mm. 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. vigilav, 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 mm. in 
length. They are more slender than the corresponding stages of 
S. penerinki, Bate, which they closely agree with 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 = 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 3 
of the length of the eye-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 the median line armed with a lobe 
produced to a spine, and in these and even in specimens c. 10 mm. 
long the posterior margin of the carapace is armed with an erect 
spine. 
