COPEPODA. 
41. 
chitin everywhere in the cephalothorax is almost covered with prickles, and the 
whole animal is more robust. „ 
MICROCALANUS (Sars). 
Microcalanus rusillus. 
(Plate II., fig. 5.) 
Microcalanus pusillus, 6. 0. Sars, Crustac. of Norway, IV. (1903), p. 156. 
Pseudocalanus pygmccus, Sars, Norwegn. N. Polar Expdn., Yol. Y. Crustacea, 1900. 
„ „ G-iesbrecht, ‘ Belgica ’ Report, p. 20. 
Sars originally described a small Calanoid, Pseudocalanus pygmseus, from Nansen’s 
Polar Expedition, which he subsequently re-named Microcalanus , and included in the 
new genus a second and still smaller form of M. pusillus. Giesbreclit described a small 
Calanoid from the Antarctic seas, which agreed generally with Sars’ Polar species, except 
for very small differences, e.g. the length of the anterior antennse, and the length of the 
outer marginal spines of the exopodites of the feet. The size varied, mostly from 
0 1 7-0’75 mm. ; some were even smaller, P. pygmseus , Sars (= M. pygmoeus,- Sars), is 
a little longer, viz. ' 86 mm. (Sars). 
The ‘ Discovery ’ collection contains many examples of a very small Calanoid 
which agrees so closely with Sars’ M. pusillus, that I regard them as identical; and 
Giesbrecht’s Pseudocalanus pygmseus must, I think, be also regarded as identical. This 
small Calanoid is one of the few examples of complete agreement in form and structure 
between the Polar and Antarctic forms, and on that ground is of interest. 
The female is 0 - 60 mm. long. Cephalo thorax two and a half times longer than 
the abdomen ; the head and first segment united, the former evenly rounded and with 
short, delicate rostrum ; the greatest breadth not cpiite half the length; the last 
thoracic segment with rounded and only slightly produced margins ; abdomen of four 
segments. The genital segment is nearly twice as long as the next, which is rather 
larger than the distal segment, and the anal as long as the segment preceding it. 
Furcal segments as long as the anal and longer than broad, with four short terminal 
bristles. The genital segment is very swollen laterally, but symmetrical, and rather 
tumid ventrally. 
Anterior antennse reaching about the end of the genital segment, and of twenty- 
three joints. 
Posterior antennse with Re about one-third longer than the Ri. 
Mandibles with Ri much longer than Re, both rami fully segmented. 
Masticatory plate with strong cutting teeth seven or eight in number, and distally 
nearly half as broad as long. 
Anterior foot jaw^s with five well-formed lobes, of Calanus type, with well- 
segmented Ri. 
