52 THE STUDY OF ANIMAL LIFE CHAP. 



which make life worth living. Moreover, though the 

 reproductive system never degenerates, the odds are 

 often many against an embryo reaching a fit host or 

 attaining maturity. Thus Leuckart calculates that a 

 tapeworm embryo has only about 1 chance in 83,000,000 

 of becoming a tapeworm, and one cannot be sorry that 

 its chance is not greater. In illustration of the degenera- 

 tion which is often associated with parasitism, and varies 

 as the habit is more or less predominant, take the case of 

 Sacculina a crustacean usually ranked along with bar- 

 nacles and acorn-shells. It begins its life as a minute 

 free " nauplius," with three pairs of appendages, a short 

 food-canal, an eye, a small brain, and some other struc- 

 tures characteristic of many young crustaceans. In spite 

 of this promiseful beginning, the young Sacculina becomes 

 a parasite, first within the body, and finally under the 

 tail, of a crab. Attached by absorptive suckers to its 

 host, and often doing no slight damage, it degenerates 

 into an oval sac, almost without trace of its former 

 structure, with reproductive system alone well developed. 

 Yet the degeneration is seldom so great as this, and it is 

 fair to state that many parasites, especially those which 

 remain as external hangers-on, seem to be but slightly 

 affected by their lazy thievish habit ; nor can it be 

 denied that most are well adapted to the conditions of 

 their life. But on the whole the parasitic life tends to 

 degeneration, and is unprogressive. Meredith writes of 

 Nature's sifting 



" Behold the life of ease, it drifts. 

 The sharpened life commands its course : 

 She winnows, winnows roughly, sifts, 

 To dip her chosen in her source. 

 Contention is the vital force 

 Whence pluck they brain, her prize of gifts." 



4. General Resemblance to Surroundings. Many trans- 

 parent and translucent blue animals are hardly visible 

 in the sea ; white animals, such as the polar bear, the 

 arctic fox, and the ptarmigan in its winter plumage, 

 are inconspicuous upon the snow ; green animals, such 

 as insects, tree-frogs, lizards, and snakes, hide among the 



