336 Records of the Indian Museum. [VoL. Vide 
Immature forms :— 
In the younger stages of development we find that the males 
and females closely resemble one another, practically the only 
difference being the presence of the 5th leg; neither of the an- 
tennae showing, as yet, distinctive male features. 
Stage 3. In both sexes the abdomen consists of four seg- 
ments having, together with the furcal rami, the following propor- 
tional lengths :—4:6:5:9: 5. 
The ist and 2nd antennae are of the female type and with 
the exception of the 5th leg in the male all the other mouth parts 
and appendages are as in the adult. 
The 5th leg of the male is shozt, reaching only to the 2nd 
basal joint of the 4th leg and consisting of a basal portion and 
three joints having the proportional lengths 6:5: 13. The last 
joint bears 3 spines, two terminally and one midway along the 
margin, at which point it also shows a slight constriction. From 
its appearance it is evident that at the next moult it will divide 
into two, to form the last two segments of the adult leg. 
There is a very close relationship between the two genera 
Acrocalanus and Paracalanus and, from the study of these imma- 
ture forms of the present species and examples of so-called adults 
of other members of these genera, it would appear that in mature 
males we get certain sexual characters in both 1st and 2nd 
antennae as well as in the 5th leg. 
In Paracalanus parvus, Acrocalanus pediger, and A. gardinen, 
as well as in the present species, in sexually mature males we find 
that (1) the rst antenna has a marked tendency towards the 
coalescence of the basal segments, (z) the distal part of the end 
segment of the exopodite of the 2nd antenna is rounded and 
nipple-like and is devoid of setae, (3) the abdomen consists of five 
fully-separate segments, and (4) the left leg consists of a basal 
portion and 4 segments, of which the terminal bears two and 
the penultimate a single spinous process. 
Males have also been described in the case of A. longicornis, 
monachus and gracilis and Paracalanus aculeatus, but in all 
cases they present only four completely-separate abdominal seg- 
ments ; the antennae are of the female type and the 5th leg has 
only three separate joints apart from the basal portion and the 
terminal joint bears three spines. I therefore entirely agree 
with Wolfenden (1906, p. 1002) that in these instances the males 
were immature and a comparison of these forms with stage 3 
of the above species shows that they are of corresponding ages. 
Both Wolfenden (1906) and Cleve (1901) have described forms 
which they took to be females in which a small 5th leg was 
present belonging to the following species, Acrocalanus longtcornis, 
A. gracihs, and A. gibber (by Wolfenden) and Acrocalanus pediger 
(by Cleve), but I am inclined to regard most, if not all, of these 
examples as immature males corresponding to stages 3 and 4 as 
described above, 
