CRUSTACEA. 65 



in the waters, though wholly unable to walk. It must 

 be remarked also how this eapacity of a locomotive 

 organism to change has enabled it to occupy places 

 where, but for this capacity, it could not have existed. 

 Thus it can live successfully upon the rocks, and anchor 

 itself securely where the surf would sweep away or de- 

 stroy the typical forms of the same class. It will be 

 seen, moreover, that the barnacle accomplishes this by 

 changing during its growth from the common Nau- 

 plius, or typical Crustacean, to a form which is difficult 

 to recognize as belonging to the same class. 



The order Copepoda is represented by the genus 

 Cyclops, found in great numbers in fresh water, and 

 often seen in our drinking water. 



The body of Cyclops (Fig. 33) is shaped somewhat 

 like a pear. In the middle of the front of the head 

 there appears to be a single eye {p y), though closer 

 observation proves that this eye is really double. The 

 two pairs of antennae (a i,a 2) are the chief organs of 

 locomotion. Following the antennae come the mandi- 

 bles and two pairs of maxilla;, and next to these the five 

 pairs of swimming-feet, most of these being invisible 

 from above. The egg-masses attached to the first 

 abdominal ring are very characteristic of the animal 

 (Fig. ^7,, eg). The similar form of the young or Nau- 

 plius form in all the Crustacea may be seen by com- 

 paring (Fig. 34) the young of Cyclops with (Fig. 36, a) 

 that of the barnacle, both of them having the three 

 pairs of appendages (a 1, a 2, and />). Fig. 35 repre- 

 sents an older stage, during which the abdominal seg- 

 ments make their appearance. See also the young of 

 Lernea branchialis (Fig. 40). 



