204 CRUSTACEA CHAP. 
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, eg. Calanus jinmarchicus, 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 con- 
genial 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, 7.e. unhindered 
by the present barriers of land. 
The Abyssal regions of the sea contain many of the most 
interesting Crustacea. Families entirely confined to the abyss 
are the Eryonidae, Pylochelidae, and certain Caridean Prawns 
(Psalidopodidae, etc.), but there are a great number of normally 
littoral genera which have representatives in deep water. If 
we draw the limit between the littoral and abyssal regions at 
about 200 metres, we can characterise the latter as absolutely 
dark except for the presence of phosphorescent organisms, 
with the temperature at a little above zero, and with a 
comparative lack of dissolved oxygen in the water. These 
conditions bring about remarkable modifications in the structure 
and life-histories of the inhabitants of the deep sea: we have 
already touched on the modifications of the visual organs and on 
the presence of phosphorescence in many of the animals; other 
points to be noticed are the usually uniform yellowish or bright red 
coloration, the frequent delicacy of the tissues without much calci- 
fication, variations in the structure of the breathing organs, e.g. in 
