168 



THE EVOLUTION THEORY 



larva of* the Sacculinae bores its way into the inside of a crab or 

 hermit-crab, at the same time losing its limbs, segmentation, and its 

 chitinous covering ; and within the body of its host it is transformed 

 into the sac-like organism we have already described. After a time it 

 emerges again on the surface, and remains attached to the abdomen of 

 its host (Fig. 112, C. Sacc), drawing its nourishment from the blood 

 which it sucks up by means of its numerous delicate roots ( W, W). 



From all this we may conclude that certain Cirrhipedes in times 

 long past adopted a parasitic habit in the Cypris-larva stage, and that 

 they gradually underwent adaptations to this mode of life, and that 

 these went further and further, until the animal was transformed 



Abd 



Fig. 112. Development of the parasitic Crustacean Sacculina carcini, after 

 Delage. A, Nauplius stage. Au, eye. I, II, III, the three pairs of appendages. 

 B, Cypris-stage. VI-^I, the swimming appendages. C, mature animal {Sacc), 

 attached to its host, the shore-crab {Carcinus mcenas), with a feltwork of fine 

 root-processes enveloping the crab's viscera, s, stalk. Sacc, body of the 

 parasite, oe, aperture of the brood-cavit}'. Abd, abdomen of the crab with 

 the anus (a), 



into the singular creature which we now see in the sexually mature 

 form. 



The same is the case with the numerous fish-parasites of the 

 order Copepoda. They all leave the egg as nauplius larvae, however 

 greatly they may be modified later on by adaptation to a parasitic 

 habit, and in them we can still observe, in the fully developed 

 animals, a whole series of grades of transformation. Thus many 

 genera, like Ergasilus, are distinguished from the free-swimming 

 Copepods only by the modification of their jaws into piercing and 

 sucking organs, and of a single pair of antennae into hooks, by means 

 of which they attach themselves to the fish on which they feed. In 

 other genera the degeneration and modification go further : the 

 antennae, the eye, and the appendages degenerate more or less, and 



