328 Records of the Indian Museum. [VOT 5 Va 
Here again we see that from stage 4, an individual may 
either attain sexual maturity (stage 2) in a single moult having a 
growth-factor of 1°45, or may, by a moult with a growth the same 
as that of the female, vzz., 1°284, reach stage 3 and then become 
mature (stage 1) by a moult with the typical factor for the male 
ae 
From the above it would appear that under certain condi- 
tions we may get two dimorphic forms in any species of Cope- 
pod. 
Dimorphism in certain groups of the Arthropoda and espe- 
cially in the Insecta is comparatively common, and several in- 
stances have been reported from the Crustacea, among which I 
may make mention of the dimorphic males in Pandalus montagui, 
Leach (= P. annulicomis). ‘This was first noted by Calman (1899) 
and has since been confirmed and explained by Wollebaek (1908). 
In this case the dimorphism affects only the Ist pleopod of 
the male and is merely a breeding and non-breeding form, that is 
to say the dimorphism is seasonal and is of the ‘‘ facultative” 
type. 
Another example of facultative dimorphism in the Deca- 
poda has been described by G. Smith (1906) in two species of 
Inachus (I. scorpio and I. thoracicus) in which the great chelae are 
affected. 
The type of dimorphism which appears to be present in 
Labidocera euchaeta, Paracalanus aculeatus, and possibly in all the 
species of the Copepoda does not, however, follow either of the 
above cases, and we have apparently a ‘“‘high’’ and a “‘low”’ 
form in both males and females. 
As I have already mentioned in the case of Labidocera euchaeta, 
it would appear that in the females growth takes place by a series 
of moults which follow ‘‘ Brooks’ Law’’ and the last two stages 
are sexually mature, thus giving rise toa ‘‘high’’ and a “ low”’ 
form, which differ from one another not only as regards size but 
also in respect to certain structural characters, which, however, 
as I shall subsequently show, are caused by a direct continuation 
of those changes which afe undergone by the individual during 
the various moults from the later Copepodid stages to the ‘‘ low’”’ 
or Ist sexually-mature form. In this respect the dimorphism 
agrees exactly with that described by Fowler (loc. cit.) as occurring 
in the Ostracoda. 
In the case of the males, however, the type of dimorphism 
is somewhat different and it would appear that the “‘ low’’ sexually 
mature form (stage 2) does not directly give rise to the ‘‘ high”’ 
form (stage 1). That is to say, the type of dimorphism is ‘‘ defint- 
tive.’’ Some factor, at present unknown but possibly seasonal, 
exercises an influence on a Copepodid form (stage 4) and deter- 
mines which of the two forms shall be finally adopted by the 
individual. 
It may possibly happen that when circumstances such as 
food supply, etc., are favourable, as in the summer season, the 
