PLANKTON OF 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 medusa?, among them the largest (Aurelia and Cyanea), likewise seek 

 the surface even in bright sunlight, while smaller species, notably the common 

 hydroid medusa Phialidium 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, probably 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 Sagittse. Since 

 most of the deep-water members of the plankton (e. g., Euchaeta, the largest of local 

 copepods, and the chaBtognath 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 chaBtognath genus 

 Eukrohnia (p. 328) ; more rarely by the larger glass worm S. lyra (p. 327) ; fre- 

 quently by the large pelagic decapodous shrimp Pasiphasa; and locally by large 

 numbers of the euphausiid shrimp MeganyctipTianes 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; 



