Chap. 29 ARTHROPODS CRUSTACEANS 587 



way stimulate movement of the pigments in the retina of the eyes (Fig. 15.2). 

 Experiments have shown that products of the sinus glands regulate shifting 

 color changes in the skin once thought to be nerve controlled, 



Entomostracans 



Entomostracans are crustaceans, most of them small, even microscopic, and 

 numerous beyond imagination. They feed upon the minute plants of fresh and 

 salt waters and thus are the chief means of turning them into food for higher 

 animals. They are themselves the main food of nearly all young fishes and 

 the adults of several market fishes. There are three groups, the branchiopods, 

 copepods, and ostracods. 



Branchiopods 



The gill-footed crustaceans, Branchiopoda, have thoracic feet that are ex- 

 panded and function as gills. Most of them live in fresh water, among them 

 the fairy shrimps (Eubranchipus), the largest and most colorful of entomos- 

 tracans but not important food producers. The most common branchiopods, 

 of the Order Cladocera, are the almost microscopic water fleas. The body, but 

 not the head, is enclosed in a bivalve shell so transparent that the pulsating 

 heart, the circulating blood, the contracting muscles, and vibrating gill feet 

 can be clearly seen. Many water fleas swim by their antennae; Daphne and 

 others with long antennae take slow strong strokes and go through the water 

 in jumps; those that have short antennae make quicker strokes and progress 

 evenly (Fig. 29.5). 



The females carry the eggs and developing young in brood sacs. In sum- 

 mer they reproduce parthenogenetically. Their possible productivity is sug- 

 gested by the calculation that, barring accident, the descendants of one female 

 Daphne pulex might reach 1 3 billion in 60 days. Their populations create liv- 

 ing soup. 



Copepods 



From springs to lakes, from tide pools to the open ocean hardly any body 

 of water is without copepods. Those of one or another species are active in 

 summer and winter, most abundant wherever there are diatoms, their main 

 food. The great populations of glassily transparent copepods, a large part of 

 the surface fauna of the ocean, are the main link in the food chain between 

 microscopic plants and large animals. The blue whale, the largest of living 

 animals, feeds chiefly upon Calenus. Two tons of this little copepod. believed 

 to be one day's swallowing, have been taken from the stomach of a blue whale. 

 Three simple eyes (ocelli) are often fused into one compound eye. The one- 

 eyed jerky copepods, Cyclops, live as well in aquaria as they do in ponds. The 

 developing eggs and sometimes the young ones (nauplii) are carried in brood 



