NUTRITION 



cnidaria. But they are very numerous among the 

 platodes. The suctorial worms (trematodes) live partly 

 externally (as ectoparasites) on other animals and part- 

 ly inside them (as endoparasites), and produce serious 

 diseases in them. They have lost the vibratory coat of 

 their free-living ancestors, the turbellaria, and acquired 

 clinging apparatus instead. The tape-worms {cestodes), 

 which live entirely in the interior of other animals, and 

 are descended from the suctorial worms, have lost their 

 gastro-canal ; they are nourished by imbibition through 

 the skin. The same degeneration is found in the itch- 

 worms (acanthocephala) among the vermalia, the para- 

 sitic snails (entoconcha) among the moUusks, and the 

 root-crabs (rhizocephala) amon'g the Crustacea. 



The class of Crustacea affords the most numerous and 

 most instructive examples of degeneration through 

 parasitism, because in this class it is found polyphyleti- 

 cally in very different orders and families, and because 

 their highly organized body shows every stage of de- 

 generation together in the different organs. The free- 

 living Crustacea generally move about very rapidly and 

 ingeniously; their numerous bones are well jointed and 

 excellently adapted for the most varied methods of loco- 

 motion (running, swimming, climbing, digging, etc.); 

 their organs of sense are highly developed. As these 

 are no longer used when they take to parasitism, they 

 atrophy and gradually disappear. The younger Crustacea 

 all proceed from the same characteristic form of the 

 nauplius, and swim freely about ; later, when they settle 

 down to parasitic habits, their organs of sense and loco- 

 motion atrophy. As Fritz Muller-Desterro showed in 

 his famous little work. For Darwin (1864), forty years 

 ago, the Crustacea afford most luminous proofs of the 

 theory of descent and selection, and of progressive 

 heredity and the biogenetic law. These facts are the 

 more important as the crab undergoes the same de- 



237 



