THE ARGUMENT FROM EMBRYOLOGY 81 



Again, we may take animals such as a prawn, a 

 barnacle, and one of those curious sac-like parasites 

 of the genus Sacculina, which are found not uncom- 

 monly adhering to the under surface of the rudimen- 

 tary tail of crabs. These three animals are, when 

 adult, very unlike one another. The prawn is a free- 

 The barnacle is fixed firmly to rock, 



swimming form 



Fig. 9. 



Stages in Development of the Prawn (Peneas). 



A, Nauplius stage ; B, Zocea stage, in which the larva resembles an 

 dult Copepod ; C, Schizopod stage, where it corresponds in structure to 

 the adult Schizopoda ; D, Adult Peneus. 



usually between tide-marks ; it forms a hard protective 

 shell, has no eyes, no locomotor organs, and is herma- 

 phrodite. Sacculina is altogether unlike the other two 

 animals : it has asoftunjointed body, no trace of limbs, 

 no mouth or alimentary canal, and no sense-organs : it 

 is, in fact, merely a soft-walled bag of eggs, attached 

 to the crab's tail by a number of branching root-like 



v 



