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TEXT-BOOK OF ZOOLOGY 



in the economy of Nature, as forming the food of many larger animals. 

 Among these small crustaceans three groups may be easily distinguished. 



ORDER V. : OAR-LEGGED CRUSTACEA (COPEPODA). 



THESE are narrow and somewhat elongated creatures, which skip 

 swiftly through the water, and form, as we have already learnt, an 

 important item in the food-supply of the herring (which see). Among 

 them we are^sure to find many (females) which carry about with them 

 one or two masses of eggs until the larvae are hatched. All are of so 

 delicate a*] texture (aquatic animals ; see fresh-water polyp) that they 

 breathe by their skin, and therefore dispense with branchial organs ; 



A FIIESH- WATER COPEPOD (Diaptomus). (Magnified about thirty times.) 

 Female with a mass of eggs attached to the first abdominal segment. 



many, even, do not possess a heart. On the front of the head, between 

 the long first pair of antennas, is situated a single median eye. Like 

 the members of the two other groups, they multiply with astonishing 

 rapidity, so that standing waters are often in a short time populated by 

 them in millions. 



ORDER VI. : BIVALVED CRUSTACEA (OSTRACODA). 



IN these minute Crustaceans the body is enclosed in a minute bivalve 

 shell, giving them the appearance of tiny bivalve molluscs. When, 

 however, the animals protrude their antennae, which they use as oars, 

 from between the two valves of the shell, we see at once that we are not 

 dealing with molluscs, but with arthropod animals. 



