© DEPARTMENT OF THE NAVAL SERVICE 
7? GEORGE V, A. 1917 
CRUSTACEA. 
The Crustacea are the most interesting group represented in the zoo-plankton, 
both on account of the number of species represented, and for the fact that, in the 
majority of gatherings, they form the greater bulk of the material. It will be con- 
venient to consider the various forms observed under their proper orders 
Cladocera. 
Representatives of this order were found much less frequently than was expected, 
occurring in any considerable numbers in only one gathering, i.e., in that taken 
October 16, from the 6-metre level. All the forms observed in this gathering were 
representatives of the species Podon polyphemoides Leuckart. 
Copepoda. 
Forms belonging to this group were the most constant constituents of the 
plankton, being found in every gathering, with one exception, and usually in con- 
siderable numbers. It is noteworthy, however, that in the spring months when 
Thalassiosira became a prominent constituent of the plankton, the Copepoda became 
very much reduced in numbers. At least this was the case so far as the surface 
water down to the 6-metre level was concerned, the Thalassiosira extending to that 
depth, but it is quite likely that the Copepoda were present in undiminished numbers 
at levels beyond those occupied by the alga. The diminution of the Copepoda in the 
surface water coincidently with the appearance of T'halassiosira is clearly indicated 
in the table if one compares the frequency records for Acartia claust’and the diatom. 
Of the members of the family Calanide, special interest attaches to Calanus 
finmarchicus (Gunner) Boeck, on account of its forming so important a constituent 
of the plankton of northern waters. It occurred at intervals throughout the winter, 
but never in any great quantity, although in several gatherings it was present in 
sufficient numbers to deserve the term “frequent.” It is to be noted, however, that 
the plankton now under discussion was collected in the immediate vicinity of St. 
Andrews, and it is quite probable that C. finmarchicus may be much more abundant 
in more open water. Herdman in 1897! found it very abundant in the gulf of St. 
Lawrence and in the Atlantic off the entrance to the straits of Belle Isle, and my 
colleague, Dr. A. G. Huntsman, obtained it in large numbers in rather deep water 
off Eastport, Me., and off Grand Manan in September, 1915. 
The much larger OC. hyperboreus Kroyer was observed in only one gathering, 
and then only as a single individual. The fact of its occurrence is, however, of 
interest as it has not previously been recorded from Canadian waters. 
A third Calanid, Pseudocalanus elongatus Boeck, easily recognized by the 
absence of the fifth pair of legs in the female, occurred in about the same degree of 
frequency as UV. finmarchicus. 
Of the family Centropagide, the genus Eurytemora furnished two representatives, 
EB. hirundoides Nordquist and EH. herdmani, Thompson and Scott. Neither was 
abundant in any gathering, but both occurred at intervals throughout the season 
represented by the collection, and were occasionally “frequent.” MTemora longi- 
cornis (Mill) Boeck also occurred at intervals in the autumn and winter until the 
end of January, after which it was not observed. On the last date on which it was 
found (January 27) it was the most abundant constituent of the plankton. 
Tt is the family Pontellide, however, that furnishes the most characteristic 
feature of the plankton now being discussed, the form concerned being Acartia 
1w. A. Herdman. On the plankton collected continuously during two traverses of the North 
Atlantic in the summer of 1897. Trans. Liverpool Biol. Soc., xii, 1898. 
