• '.r?- 



*l 



t» ^ 





m 



A. 





riasi** 



The marine copepod Calanus finmarchicus is only the size of a large grain of rice. This abun- 

 dant crustacean makes amazing daily migrations between depths of 1500 feet and the topmost 

 150 feet of water. Also shown are a few, smaller cladoceran Evadne. (England. D. P. Wilson) 



when freshly caught, is found to have small, flattened 

 crustaceans scurrying over its surface. Others may 

 be discovered clinging to the gills or the lining of the 

 mouth, taking nourishment through the thinner, softer 

 parts of the fish's skin. These are likely to be Ar- 

 giiliis, the fish louse, a copepod with two prominent 

 suckers on the under surface. Still other copepods go 

 through an ordinary nauplius stage and then attack a 

 fish, embedding themselves and growing larger as al- 

 most formless parasites. Yet when the female para- 

 sitic copepod produces eggs, she extrudes them in 

 two egg pouches suggesting those of tiny, independ- 

 ent Cyclops. 



OSTRACODS 



The nauplius stage found in copepods appears to 

 be an important first step in the life history of many 

 crustaceans, both small and large. It is the first free- 

 swimming stage for members of order Ostracoda, 

 creatures a microscopist is likely to encounter while 

 examining a bit of oozy mud from the bottom of a 

 pond or a fragment of the floating algal mat at the 

 surface. Ostracods have an almost egg-shaped, bi- 

 valved shell rarely more than ' , ,; of an inch in 

 length, hinged at the back and capable of being closed 

 entirely by contraction of a special muscle. These 

 crustaceans are omnivorous scavengers which climb 



232] 



