CRUSTACEANS 



225 



Classification. — All the forms of crustaceans mentioned belong to 

 that subclass of crustaceans called the Malacostraca, the characters 

 of the group being a definite number of segments and appendages. 

 All having five pairs of walking appendages are called decapods. 

 How many of the above-mentioned forms are decapods? 



Entomostraca. — Another subclass qf crustaceans, in which the number 

 of appendages is not fixed, is the group Entomostraca. They arc mostly 

 small animals, some species existing in countless numbers. Such are the 

 fairy shrimps found appearing in early spring in fresh-water ponds, little 

 translucent swimming forms from one half to 

 three fourths of an inch in length. Another 

 fresh-water form often seen in aquaria is the 

 water flea (Daphnia). From the economic stand- 

 point, probably the most important crustaceans 

 that we shall study are the copepods. These 

 tiny animals are barely visible to the naked eye. 

 They are found in almost every part of the 

 world, from the arctic seas to those of the tropics, 

 and in fresh as well as salt water. They are 

 so numerous that the sea in places is colored 

 by their bodies. So prolific are they that it is 

 estimated that one copepod may produce in a 

 single year four billion five hundred million 

 offspring. These animals form a large part of 

 the food supply of many of our most important 

 food fishes as well as the food of many other 

 aquatic animals. They are, then, in an indirect 

 way, of immense economic value. 



Degenerate Crustaceans. — One of the most 

 interesting forms to a zoologist is the goose bar- 

 nacle. This crustacean is free-swimming during its early life. Later, 

 however, after passing through several changes in form during its de- 

 velopment, the barnacle settles down on a rock or some floating object, 

 fastens itself along the dorsal surface, and remains so fastened during the 

 rest of its Kfe. Food comes to it in a current of water, which is set in 

 motion by the rhythmical beating of the appendages. Thus food particles 

 are carried along the ventral side of the body to the mouth. Such animals 

 are said to be degenerate. 



Parasitic Crustaceans. — Other crustaceans have become even more help- 

 less and have come to take their hving from other animals. In some cases 

 they are simply a bag for absorbing nourishment from the host on which 

 they are fastened. Such is the Sacculina, a degenerate crustacean that Hves 



hunter's BIOL, 16 



Cyclops, a common cope- 

 pod, enlarged about 

 twenty times. A mass 

 of eggs at the right. 



