412 ESSENTIALS OF ZOOLOGY 



into two groups, with the legs of each group pointing toward each 

 other. These are of particular value as fresh- water fish food. 



Entomostraca as a group is composed of many smaller crustaceans 

 occurring in great numbers in both marine and fresh waters. The 

 fairy shrimps (Euhranchipus) are delicate, transparent and feathery 

 appearing. They are about three-fourths of an inch in length. They 

 swim with the ventral side up and their long, leaf-life appendages 

 hang from the body. These appendages serve also as respiratory 

 organs. They live in cool streams in the spring and fall, but the 

 summer is passed in the egg, which can withstand complete dryness. 

 Many of them are parthenogenetic, hence, males are rare. The com- 

 mon marine form is Artemia, often called brine shrimp. 



The water fleas including Daphnia of order Branchiopoda, Cyclops 

 and Diaptoynus of order Copepoda and other small Crustacea con- 

 stitute an important common group. Daphnia is one that is en- 

 closed in a delicate bivalve shell. The second pair of antennae are 

 very large and are used in swimming. The shell is beautifully 

 marked and terminates in a caudal spine. They are only about 

 one-tenth of an inch in length. Cyclops is another common fresh- 

 water form with the antennae shorter than the cephalothorax whose 

 body length is also about one-tenth inch. It has a single median 

 eye, and the females frequently are seen with a pair of egg sacs 

 attached at the base of the abdomen. Diaptomiis, another Copepod, 

 is a common form of about the same form and size as Cyclops, except 

 that the antennae are nearly as long as the body. Arguhis is a genus 

 of Copepods w^hich is parasitic on fish, and the individuals are 

 called fish lice or carp lice. They are flat creatures and are found 

 running around over the scales of their hosts. Some of the other 

 forms are parasitic on the gills and fins of fish and their bodies be- 

 come greatly modified. 



The ostracods are small, swimming, bivalve forms that are some- 

 times called swimming clams. This group has beautifully marked 

 valves; in fact, these animals are the most beautiful found in the 

 plankton. 



Adult barnacles of order Cirripedia bear so little resemblance to 

 other Crustacea that they are usually overlooked as such by the 

 layman. They are completely encased in. a thick shell of several 

 sections and have the general appearance of an oyster or clam. 

 They are sessile in habit as adults, though free-sv/imming in the 

 larval stage. Their entire life is spent in marine waters. There 



