THE DAPHNTA ANT) ITS ALLIES 



129 



one Cyclops might, under the most favorable conditions, 

 have 5,000,000,000 descendants in one year. It is conse- 

 quently easy to understand how Cyclops often becomes 

 the most abundant entomostracaii in our waters, and how 

 in some lakes it has been found that there are over one 

 million of them to each square metre of water surface. 

 Large numbers of the Copepoda are marine. One of the 

 most common is Acartia (Fig. 119), which swarms to such 



FIG. 120. Mussel-shell bearing barnacles (Balanus). Photo, by W. H. C. P. 



an extent on the surface of the water as to make great 

 phosphorescent areas. 



Barnacles are the only attached non-parasitic Crustacea. 

 Certain species of them are found fastened to rocks on the 

 seashore at low-tide mark. If you watch barnacles in rock 

 pools, you can see them open the valves of their shells, 

 protrude their elongated appendages, which together form 

 a sort of rake, and pull in particles which happen to be float- 

 ing about them. Other species of barnacles attach them- 

 selves to floating seaweed, ship bottoms, and whales ; under 

 these circumstances, despite their sessile habit, they enjoy a 

 constant change of locality. Barnacles doubtless gain 

 great protection from the circumstance that they are 



