THE ARTHROPODS 



111 



Crustacea, and in several points resemble the ancestral form 

 from which all the modern species have descended. Some 

 nearly related forms are provided with a great fold of the 

 hody-wall, which may almost completely conceal the animal 

 from above, or it may be formed like a bivalve clam-shell, 

 within which the entire body may be withdrawn. This 



FIG. 59. Fairy-shrimp (Branchipus). b, brood-pouch ; e, e', 

 compound and simple eyes ; /, paddle-shaped feet ; A, tu- 

 bular heart ; i, intestine. 



latter character is also found in the water-fleas (Dajphnia), 

 very much smaller forms, and sometimes occurring in mil- 

 lions on the bottoms of our ponds and marshes. They are 

 readily distinguished from the fairy-shrimp by the short- 

 ness of the body, the small number of appendages, and by 

 their habit of using their antennae as swimming organs, 

 which gives to their locomotion a jerky, awkward character. 

 106. Cyclops and relatives. Cyclops (Fig. 60), the repre- 

 sentative of a number of lowly forms belonging to the order 

 of CopepQds, is one of the commonest fresh-water Crustacea. 

 The forward segments of the spindle-shaped body are cov- 

 ered by a large shield or carapace, the feet are few in num- 

 ber, and, like its fabled namesake, it bears an eye in the 

 center of the forehead. Nearly related species are also re- 

 markably abundant at the surface of the sea, at times occur- 



