ORIGIN AND RADIATION 



are squat and flattened, looking like minute isopods, and some have 

 peculiarly developed limbs. Some harpacticids creep about on the 

 surfaces of plants, others are semi-parasitic, but the majority live 

 in mud or between sand grains. 



This mode of life has been adopted by another smaller order, 

 the Mystacocarida (fig. 8a). These have narrow bodies like some of 

 the harpacticids, but the limbs are very different so that they are 

 regarded as a separate group. 



B 



Fig. 8. A, Derocheilocaris remanei (Mystacocarida). 

 Adult female, dorsal view. Actual length about 0.38 

 mm. (After Noodt, 1954). B, Hutchinsoniella macra- 

 cantha (Cephalocarida) Dorsal view, the limbs are 

 hidden by the body. The setae on the caudal rami 

 extend to about half the length of the body. Actual 

 length of body about 2.8 mm. (After Sanders, 1955). 



A totally different way of life has been taken up by the cirripedes. 

 The adult barnacles live attached to rocks and various other objects, 

 and their whole anatomy has become modified to this end. The 

 relation between the anatomy of a stalked barnacle, such as Lepas, 

 and the anatomy of a more normal crustacean is illustrated in 

 fig. 9, which has been adapted from the ingenious figure given by 



