12 THE WONDER OF LIFE 



man, and so one might continue for a long time. Among 

 Invertebrates, the unicellulars or Protozoa form a sub- 

 kingdom by themselves ; the Sponges and Stinging Animals 

 ring the changes on the possibihties of radial symmetry ; 

 Worms present a bewildering variety of types with httle in 

 common save the general tendency to be ' worm-like ' ; 

 Echinoderms, though a very well-defined series, show aston- 

 ishing contrasts, — between brittle-star and sea-urchin, 

 between the sausage-hke sea-cucumber and the graceful sea- 

 hly. The two other great series are the Arthropods and the 

 Molluscs, sharply contrasted at almost every point. Among 

 Molluscs we may compare oyster with cuttlefish, slug with 

 nautilus, land-snail with ' sea-butterfly ' ; it is difficult 

 for the ordinary observer to understand on what grounds 

 such dissimilar forms can be united under one title. Simi- 

 larly, the exceedingly successful Arthropod series, rivalled 

 only by the Vertebrates, includes Crustaceans, Centipedes, 

 Insects, Spiders, Scorpions, Mites and many other very 

 divergent types. If we consider Crustaceans, we get the 

 same impression, — water-fleas and lobsters, fish-Kce and 

 land-crabs, sand-hoppers and barnacles — what a variety of 

 form ! The crowning illustration is surely among Insects, 

 where within a relatively narrow range we find an astonish- 

 ing wealth of style, — the squat bug and the lank walking- 

 stick insect, the heavy beetle and the dainty midge, the 

 butterfly and the flea, the mosquito and the cockroach. It 

 has also to be remembered that there are many less famihar 

 types of animal life which represent quite distinct Hnes of 

 their own, such as Rotifers, Polyzoa, and Brachiopods, 

 which greatly increase the range of diversity of forms. 



Variety of Bread -Winning. — In illustration of variety 

 of habit, let us recall for a moment the variety of food- 



