CRUSTACEA. 67 



ing on the food accumulated by the surfaces of the 

 gills, loses its power to make a hard, thick shell, and 

 becomes an albino, being colorless. The terminal sec- 

 tions of the legs are hooked, and used to hold on with 

 instead of for walking, and the arms are short and 

 weak, as is suitable to their style of feeding. It does 

 not seem to injure the oyster materially, though it 

 sometimes causes a slight distortion of the gills. It is 

 not a true parasite, feeding on the blood or juices of 

 the oyster, but a sort of companion, a commensal para- 

 site. The males are generally free and very rare, and 

 females alone are found with the oysters. 



The genus Branchipus (Fig. 39), belonging to the 

 Hranchiopoda, is found in great numbers in rain-pools 

 and fresh-water ditches in the early spring. It is a very 

 curious animal, and interesting to children on account 

 of its habit of swimming with its back downward, and 

 the oar-like appendages with which it moves along. 



Specimens of the order Ostracoda are found fre- 

 quently in our fresh-water ponds, but these are too 

 small to be studied by the unaided eye. They closely 

 resemble small bivalve shells, the similarity being due 

 to the shape of the carapace, which also sometimes has 

 lines of growth like those of shells.* 



It only remains for us to compare the two great 

 classes, Worms and Crustacea. In the higher worms, 

 such as we have used to illustrate the group of Ver- 

 mes, we find an elongated body, the two ends of which 

 are often similar in form, though distinct in function. 



* Kxcellent figures and descriptions of these can he found in 

 Morse's booh, already cited- 



