THE DRIFTING SWARMS 



creatures, many of whom have mouths so small that they 

 cannot swallow anything bigger than two- or three- 

 thousandths of an inch across. For these reasons not all 

 planktonic animals have been watched through their 

 entire life cycles — and many varieties still remain un- 

 classified, their habits and histories almost completely 

 unknown to us. 



One particular species of planktonic animal persists in 

 baffling zoologists. Fishermen's nets occasionally bring 

 up single specimens of this transparent, spherical animal, 

 which has been named planktosphaera because of its 

 shape. All attempts to fit it into any known animal group 

 have failed, although it suggests relationship with several. 

 For fifty years a solution of the problem has been sought, 

 but (although it has been argued that the creature is a 

 young crinoid or sea-lily) the half-inch ball of living sub- 

 stance has not been finally classified. 



The copepod crustaceans vary considerably in size and 

 structure, but the typical copepod is divided into the 

 usual three regions of the crustacean-head, thorax and 

 abdomen. The pear-shaped head and forepart carries six 

 pairs of appendages, modified into a sensory and feeding 

 complex. It has two antennae — very long and with many 

 joints — ''arms" which, if the creature is imagined in an 

 upright position, hang down from its ''shoulders" to 

 below its forked or branched tail. Behind the head each 

 of the first five thoracic segments has a pair of jointed 

 and forked swimming legs, the movements of which 

 carry the copepod through the water in spasmodic hops 

 along what seem to be purposeful courses. The best 

 mental picture of a copepod is that of a creature which 

 looks something like an ant with no head, yet with a 

 branched tail and two extra-long arms something like 

 earwigs and a number of legs on either side of the fore- 

 part of its body which it uses in a series of jerks to propel 

 itself through the water. 



Many of the copepods look like small fleas ; others are 



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