THE INVERTEBRATE LEGIONS 



IB 



called "krill." Man, when he speaks of possible future food from the sea in 

 terms of "plankton cakes," usually means the small planktonic crustaceans which 

 are extremely abundant in polar and boreal seas in the warm months. 

 The several groups of crustaceans are briefly reviewed below. 



WATER FLEAS: Order Cladocera 



Water fleas are enclosed in a large, round, bivalve carapace, which does not, 

 however, cover the head, and have two large antennas which are organs of 

 locomotion. Daphnia is a fresh-water form commonly sold in pet shops as a 

 living fish food. Some water fleas are verv oddly shaped, and all are small, 

 scarcely the size of the head of a pin. Their distribution is world-wide. They 

 are planktonic, very numerous, and an important food for plankton-eating fishes. 



OSTRACODS: Order Ostracoda 



Like water fleas, ostracods are planktonic, small, swim with the antennas, 

 and are enclosed in a bivalve shell. But ostracods look just like little swimming 

 clams. The study of ostracods is especially important to oil prospectors because 

 certain fossil ostracods are found associated with oil deposits. 



NAUPLIUS LJ\RVA 



Fig. 37. Lower crustaceans. Adapted from various authors including the Reports of 

 H.M.S. Challenger Ql 873-7 6') and drawn from life. 



