COPEPODA 745 



second stage (Fig. 1173) a fourth pair of appendages is added, 

 which later are known as the maxillae. In a later stage three more 

 pairs of appendages are added, the maxillipedes, and the first two 

 pairs of swimming feet: this is known as the metanauplius stage. 

 The following stage is the first Cyclops stage; in this there is a dis- 

 tinct division of the body into cephalothorax and abdomen, and the 

 third and fourth swimming feet are present in a rudimentary form. 

 In this stage, too, the anterior appendages have developed into 

 forms more similar to those in the mature animal. 



The process of development is thus accompanied by a continued 

 increase in the number of appendages beginning at the anterior 

 extremity, in the number of segments of the cephalothorax and 

 abdomen, and in the complexity of the appendages, until the mature 

 forms are reached. 



Some of the parasitic forms do not pass through all these stages. 

 There are some that never acquire the third and fourth swimming 

 feet; in others, by a progress of regression, the first and second 

 feet may disappear. Some parasitic forms jump the whole series 

 of nauplius stages and almost immediately after leaving the egg 

 appear in the first Cyclops stage. 



Hardly any body of water is without its copepod inhabitants, 

 although running waters have a less abundant population than 

 lakes. Frequently standing pools swarm with the individuals of 

 one or a few species of this order. Temporary pools in the spring, 

 which are formed in the same place in successive years, will some- 

 times be almost literally filled with Copepoda, which are strictly 

 seasonal in their life habits; for, as the pools disappear, the cope- 

 pods disappear, their eggs sink in the mud of the bottom, and 

 remain until the waters of the next season bring about favorable 

 conditions for their generation. 



The lakes produce an exceedingly abundant copepod fauna, 

 which has an important practical interest, for the ultimate food of 

 fish is composed almost entirely of these organisms; that is, the 

 small fish of our most abundant species feed entirely upon Ento- 

 mostraca, of which Copepoda form the greater part, and, in many 

 cases, the mature fish also feed entirely on these same minute 

 creatures. 



