106 NATURE STUDY REVIEW [9 :4— April, 1913 



Fig. 7 and 8, the females of this group carry their eggs in 

 ovisacs. In the Cyclopidae family this sac is double, while in 

 the other two groups it is usually single. 



The ostracods (Fig. 9 and 10) are easily mistaken for small 

 bivalve molluscs in as much as the body is entirely covered by a 

 bivalve shell, closed by powerful muscles. However, the shell is 

 not made of lime but of chiton, and a close examination shows 

 that jointed appendages extend from the body, some of which 

 may be used for swimming or craw^ling. Here as in the copepods 

 the major swimming organs are the antennae, which are pro- 

 vided with a number of long hairs, forming a dense brush, which 

 acts as a paddle. In size the ostracods vary from about a fourth 

 of an inch in diameter to mere specks in the water, scarcely ap- 

 parent with the naked eye. Almost all of the ostracods known 

 in North America belong to one family, the Cyprididae. The 

 genera are hard to distinguish even with a microscope so that it 

 is sufficient merely to be able to recognize them as ostracods. 



The cladocera (Fig. 11, 12, 13) have a shell which consists 

 of two thin plates that usually do not cover the entire body and 

 sometimes are reduced to small plates near the posterior side and 

 may be embedded under the external covering. They have sev- 

 eral pairs of leaf-like feet and, as in copepods, there is a single 

 median eye. The antennae also serve this group as swimming 

 organs. The cladocerans ordinarily collected may be as large as 

 one-eighth inch in diameter though they are usually much smaller. 

 There are several kinds commonly taken, but since these are not 

 as easily distinguished as is the case with the copepods, no descrip- 

 tion of them will be attempted. Some of the different types may 

 be seen in Fig. 11, 12, and 13. 



These smallest crustaceans, like all the others considered 

 here, are to be found as soon as the ice begins to thaw in the 

 spring and continue abundant throughout the summer season. 

 They may be found 'in Lake Michigan, in any park lagoon, or 

 in any pond that lasts a month or more. The ostracods are more 

 often found on the bottom and are more common proportionate- 

 ly in temporary ponds. Cladocera reach their best develop- 

 ment in smaller permanent ponds, while copepods are dominant 

 forms in the larger bodies of water and in temporary ponds. 



The number of these small animals that may be present in 

 the water is rather surprising. Thus from representative counts 

 taken from a standard collection from Lake Michigan it has been 

 estimated that over 5,250 copepods alone are present in 100 liters 

 (approximately 28 gallons) of lake water. It is estimated that 



