204 CRUSTACEA 



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 lie 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 finmarchiciis, behave in 

 the converse manner, preferring tlie 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 circum tropical, i.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 Ijring al)Out remarkable modifications in the structure 

 and life-histories of the inliabitants 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, tlie frequent delicacy of the tissues without much calci- 

 fication, variations in the structure of the breathing organs, e.g. in 



