184 ORGANIC EVOLUTION 



how, by its changed character, the animal in question is 

 more perfectly adapted to the environment it has chosen, 

 and is thus benefited. 



Among the simpler Crustacea, in the same group with 

 the common ship-barnacles and goose-barnacles, there is a 

 genus of parasitic forms called Sacculina. These are fre- 

 quently parasitic upon the common crab. When seen upon 

 the crab they appear to be little more than soft bags full 

 of eggs, and no one would suppose that they were in reality 

 Crustacea and related to the crab itself (Plate 101, C. Sacc.}. 

 They show no hard outer covering, such as is seen in all 

 normally developed Crustacea, and from which the group 

 derives its name. They have no jointed legs as do other 

 Crustacea. There is nothing in their adult anatomy to 

 suggest that they are Crustacea. No one would think for 

 a moment of so classifying them, were it not for their 

 embryology, which clearly shows that they are descended 

 from forms which closely resemble goose-barnacles. In the 

 course of their embryology we see a larva, which is like that 

 usually found in the Crustacea, the so-called Nauplius 

 (Plate 101, A). This is followed by another stage in 

 which we see the animal resembles Cypris, one of the 

 Ostracoda, a group of lowly developed Crustacea (Plate 

 101, B\ Soon the little Sacculina larva passes through 

 this stage and comes to a higher condition when it is practi- 

 cally a little goose-barnacle. Now it leaves its independent, 

 free-swimming life and becomes attached to a crab, or 

 occasionally some other animal (Fig. 46, A\ Living at- 

 tached to the crab, as it does, the parasite has no use for 

 legs or any locomotor organs, and these are cast off. Sense 

 organs are not needed, and these are lost. There being no 



