48 The Study of Animal Life PART i 



there is another aspect of the case. Parasitism tends to be 

 followed by degeneration of appendages, food - canal, 

 sense-organs, nervous system, and other structures, the 

 possession and use of which make life worth living. More- 

 over, 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 I 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 structures 

 characteristic of many young crustaceans. In spite of this 

 promiseful beginning, the young Sacculina becomes a para- 

 site, 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 unprogress- 

 ive. 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 

 transparent and translucent blue animals are hardly 



