20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I3I 



were all ambulatory limbs. Moreover, since the legs of Limnocythere 

 can be correlated with appendages of a very different structure in 

 Other genera, the ostracods give an actual demonstration of the 

 changes that legs can undergo in adaptation to new uses. The very 

 unleglike fifth appendage of Cypris (D, 5), for example, must have 

 been evolved from a typical leg such as its representative in Limnocy- 

 there (A, 5). This same appendage in Philomedes (C, 5) is again 

 quite different from the corresponding appendage in Cypris. The 

 maxilla of Cypris (D, ^) is represented in Philomedes (C, F, ^) by 

 an appendage still suggestive of its leg origin. The fifth appendage 

 of Philomedes (C, G, 5), however, has no resemblance to its counter- 

 part in Limnocythere (A, 5), though the presence of three small 

 marginal lobes and an apical tooth might be taken as evidence of a 

 former segmentation. The sixth appendage (C, 6) has likewise three 

 small marginal lobes and a broad, fringed apical lobe, but otherwise 

 it has departed far from the structure of a leg (A, 6). The seventh 

 appendage of Philomedes (C, 7) has lost all semblance of a leg; it 

 has become a long, flexible, vermiform cleaning organ armed with 

 an apical brush of recurved bristles. The corresponding appendage 

 in Cypris (B, 7)is likewise used for cleaning the shell chamber, but 

 the only concession it has made to its function is an inversion of 

 position. The sixth appendage of Cypris testiidinaria (fig. 5 E) looks 

 like a typical 7-segmented crustacean limb, counting the long terminal 

 claw as the dactylopodite. The fourth and fifth podomeres of this 

 appendage, however, are perhaps not true segments, since in Cypridop- 

 sis vidua Kesling ( 1951, fig. 20) shows that the muscles from the sixth 

 podomere have their origins in the base of the fourth podomere. 



The ostracods give no support to the theoretical phyllopod origin 

 of crustacean limbs, and show clearly how simple segmented legs 

 can be modified into very unleglike structures. 



COPEPODA 



The copepods include marine and fresh-water free-swimming spe- 

 cies and a large number of parasitic species. They are nearly all very 

 small crustaceans, mostly from 0.50 mm. to 10 mm. in length in the 

 adult stage. The simpler free-swimming copepods seem to approach 

 more closely the typical shrimplike form of the higher crustaceans 

 than do any of the other entomostracans. The body of a generalized 

 form such as the marine Calanus (fig. 6) is divided into a cephalo- 

 thoracic region bearing the appendages, and a slender limbless ab- 

 domen. The cephalothorax includes an anterior unsegmented part 



