Common oceanic species, found chiefly in the open Atlantic and in the North Sea, are _Anomalocera pattersoni_, _Calanus finmarchicus_, _Centropages typicus_, _Metridia lucens_, _Oithona plumifera_, etc. Common neritic species in the Channel and other coastal waters are _Centropages hamatus_, _Euterpe acutifrons_, _Oithona nana_, _Temora longicornis_, etc. It was found by Gough[161] that although the true oceanic species invade the Channel from the open Atlantic to the west, they become rarer and rarer as they advance up the Channel. Thus the plankton midway between the Lizard and Ushant at all times of year is about 70 per cent. oceanic, while at the line drawn from Portland to the Cap de la Hague it is about 35 per cent. Seasonal changes in the salinity of the Channel water, chiefly due to the influx of oceanic water from the Atlantic, as observed by Matthews,[162] do not clearly influence the distribution of oceanic and neritic forms. The influx of highly saline water from the Atlantic was most marked during the winter months up to February. From February to May the highly saline water receded, and during the summer months at the line drawn between Portland and the Cap de la Hague the salinity was rather low. This was increased in November by a patch of oceanic water being cut off from the main mass and passing up Channel, and it is noteworthy that during this month the highest percentage of oceanic forms was taken in the plankton of this region. _Calanus finmarchicus_ affords a clear instance of the way in which the plankton may be carried about for great distances by means of currents. This species has its home in the subarctic seas, but is carried down in the spring by the East Icelandic Polar stream to its spawning-place south of Iceland; the enormous shoals produced here are carried back, continually multiplying, along the coasts of Norway during the summer and autumn. Besides these great migrations, the plankton organisms perform daily movements, the majority of the Crustacea avoiding the surface during the day, and often going down to as much as seventy fathoms or more, and only coming up to the surface at night. Others, however, e.g. _Calanus finmarchicus_, behave in the converse manner, preferring the sunlit surface to swim in. Owing to their dispersal by means of oceanic currents the pelagic Crustacea do not offer any very striking features in regard to their distribution, and the possibility of always finding congenial temperatures by passing into the upper or under strata of water enables them to live in almost all seas. The tropical species of Sergestidae are mostly circumtropical, _i.e._ unhindered by the present barriers of land.