1912.] R. B. SEYMOUR-SEWELL: Surface-Living Copepoda. 3.29 
Copepodid stage can become at once sexually mature and thus 
give rise to the ‘‘ low” form (stage 2). Whereas, in unfavourable 
surroundings an extra Copepodid stage is passed before the 
individual attains to the “‘high”’ form (stage 1). At any rate, 
if any reliance can be placed on “ Brooks’ Taw,” it appears 
probable that stage 1 never develops direct from stage 2. A 
similar type of dimorphism has been described by G. Smith 
(1904) as occurring in a species of Isopod, Gnathia maxillaris. 
In this case, starting from a normal segmented larva, we may get 
the course of development passing through a small ‘ praniza’ 
(I—4 mm. in length) to a small adult (= the ‘‘low”’ form), or on 
the other hand we may get a larger ‘ praniza’ (= 5—8 mm. in 
length) which finally gives rise to a large adult (= ‘“‘high”’ form), 
and in this case also it appears certain that the one form is never 
derived direct from the other. 
In the only two species of Copepoda in which I have been 
able to investigate all the later stages of the life cycle, viz., 
Labidocera euchaeta and Paracalanus aculeatus, the males follow an 
exactly similar course, and the results obtained in the case of the 
other species which I have investigated tend to show that this 
condition of affairs is universal. 
We may summarize these changes in the male Copepod as 
follows :— 
Stage IV (immature). 
Stage III (immature). 
Stage!IT 
low form of &. 
Stage I 
high form of ¢. 
In all cases where they have been obtained, examples of stage I 
were much less common than those of stage 2 or 2) atid. it 
appears probable that only a few individuals undergo this final 
moult. 
Finally it would appear at least possible that in many in- 
stances so-called ‘‘species’’ of Copepoda, which are at present 
believed to be distinct, are in reality merfely high and low forms 
of one and the same species, and further investigation may 
necessitate a linking of pairs of species, as is the case at present in 
the Ostracoda (Halocypridae). 
One extraordinary feature in the above dimorphism of the 
males is the sudden assumption by certain individuals from stage 4 
of the female growth-factor, 
