PLANKTON OP THE GULF OF MAINE 
29 
most striking instances of vertically stratified plankton we have ever encountered 
resulted from a swarming of large appendicularians (fig. 25) on the surface and down 
perhaps to 40 or 50 meters over the southern edge of Georges Bank on May 17, 
1920 (station 20129), overlying a moderately abundant Calanus and young euphau- 
siid community in the deeper strata down to about 100 meters (fig. 26). 
Various medusae, among them the largest (Aurelia and Cyanea), likewise seek 
the surface even in bright sunlight, while smaller species, notably the common 
hydroid medusa Phiaiidiura languidum, sometimes swarm there in such numbers 
as to fill our tow nets to the brim. In fact, the latter seldom, if ever, sinks more than 
a few meters deep. Ctenophores, too, of several species, come up to the top on 
smooth days, where they can be seen drifting along like crystal balls (p. 372), and on 
occasion even the large euphausiid shrimps may swarm on top of the water, day as 
well as night, probabfy to avail themselves of a particularly succulent food supply; 
in the Eastport region, for instance, in summer (p. 147), and in the Isles of Shoals- 
Boon Island region in spring (p. 145), though they are no more characteristic of the 
superficial layers elsewhere and at other seasons than are the adult S'agittse. Since 
most of the deep-water members of the plankton (e. g., Euchseta, the largest of local 
copepods, and the chsetognath Eukrohnia hamata ) have occasionally been taken on 
the surface in the Gulf of Maine (pp. 235, 328), any number of this faunal group 
may be expected to appear at that level occasionally. 
It needed very few hauls from the deep trough of the gulf to show that there 
is a decided cleavage in composition between the zooplankton of the upper and of 
the lower water layers, with the 100 to 150 meter level roughly delimiting the two. 
No hard and fast line can be drawn between these communities, for the gap is bridged, 
on the one hand, by such occasional excursions of the deep-water dwellers upward 
even to the surface as have just been mentioned and, on the other, by the 
presence of Calanus, Metridia, Thysanoessa inermis, Tomopteris, Sagitta elegans, 
Euthemisto, Limacina, etc., in decreasing numbers right down to the bottom, even 
in the deepest parts of the gulf, a fact demonstrated by the closing-net hauls listed 
below (p. 50). Nevertheless, the two communities are so characteristic in general 
aspect that it is usually possible to tell at a glance whether any particular sample 
came from much above or far below 100 meters. The features making this possible 
are the abundance and regular occurrence of Euchseta norvegica in the deep basin of 
the gulf. This copepod is so much larger than any of its relatives and is made so 
conspicuous by the blue egg clusters of the female that it gives a distinctive appear- 
ance to the entire catch. It is regularly accompanied by the chsetognath genus 
Eukrohnia (p. 328) ; more rarely by the larger glass worm S. lyra (p. 327) ; fre- 
quently by the large pelagic decapodous shrimp Pasiphsea; and locally by large 
numbers of the euphausiid shrimp Meganyctiphanes norvegica (the latter, however, 
occurring in shallow water also). On the other hand, this “Euchseta” community 
includes only a sparse representation of Euthemisto, Calanus, or Pseudocalanus, 
and practically no Pleurobrachia or pteropods. 
Unfortunately we have made only one successful closing-net haul deeper than 
100 meters during all our summer cruises, for it was not until the spring of 1920 that 
our closing apparatus for horizontal hauls was developed to a dependable state; 
8951—28 3 
