206 



THE THEORY OF EVOLUTION 



only the embryonic development which now gives a 

 clue to the systematic relationship for instance with 

 Sacculina carcini, the female of which finally becomes 

 an egg-bag which pervades the whole body of its host 

 animal (a crab) with a cotton-like web. The embryonic 

 development on the other hand is that of the Cirripedia 

 (Figs. 40 and 41). From all this we conclude, and, as 



FIG. 40. CIRRIPEDIA. Sacculini carcini (after 

 Delage). 1. Second Nauplius stage. 2, 3. The 

 same after attachment of the breast piece and 

 loss of tail. 4. The bottle-like stage passing 

 into the interior of the crab. 5. The final 

 stage. 



Letter reference: an, antennae; an, eye; c, 

 tail ; cp, anus ; kr, main shield of the crab ; r, 

 the cell tube of the Sacculina larva penetrating 

 the host ; s, Sacculina externa ; z, central cell 

 mass. (After von Graff.) 



it appears to us, with entire right, that the Lepas species 

 were formerly free-swimming crabs which subsequently 

 adopted a sessile existence. Their embryogeny alone 

 and the still remaining crab characters show us clearly 

 the true nature of the Barnacles. The whole group 

 of the Cirripedia behaves in a similar manner. Very 

 remarkable beings are also the animals known as 

 Parasitica which as parasites live especially on fish, 

 upon whose skin or gills they attach themselves by 



