ARTHROPODS. CLASS CRUSTACEA 95 
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 
body-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within which the entire body may be withdrawn.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. 53.—Fairy-shrimp (Branchipus). 6, brood-pouch ; e, é’, 
compound and simple eyes ; f, paddle-shaped feet ; h, tu- 
bular heart ; 4, intestine. 
latter character is also found in the water-fleas (Daphnia), 
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 antenne as swimming organs, 
which gives to their locomotion a jerky, awkward character. 
96. Cyclops and relatives.— Cyclops (Fig. 54), the repre- 
sentative of a number of lowly forms belonging to the order 
of Copepods, 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- 
