10 The Study of Animal Life part i 



and lower, for the various classes have progressed in very 

 different directions. We may liken the series to a school 

 in which graded standards have given place to classes which 

 have " specialised " in diverse studies ; or to a tree whose 

 branches, though originating at different levels, are all strong 

 and perfect. Of the shelled animals or Molluscs there 

 are three great sub -classes, («) the cuttlefishes and the 

 pearly nautilus, {b) the snails and slugs, both terrestrial and 

 aquatic, and {c) the bivalves, such as cockle and mussel, 

 oyster and clam. Simpler than all these are a few forms 

 which link molluscs to worms. 



Clad in armour of a very different type from the shells 

 of most Molluscs are the jointed-footed animals or Arthro- 

 pods, including on the one hand the almost exclusively 

 aquatic crustaceans, crabs and lobsters, barnacles and 

 " water-fleas," and on the other hand the almost exclusively 

 aerial or terrestrial spiders and scorpions, insects and centi- 

 pedes, besides quaint allies like the "king-crab," the last of 

 a strong race. Again a connecting link demands special 

 notice, Peripatus by name, a caterpillar- or worm -like 

 Arthropod, breathing with the air-tubes of an insect or 

 centipede, getting rid of its waste-products with the kidneys 

 of a worm. It seems indeed like "a surviving descendant 

 of the literal father of flies," and suggests forcibly that 

 insects rose on wings from an ancestry of worms much as 

 birds did from the reptile stock. 



Very different from all these are the starfishes, brittle- 

 stars, feather-stars, sea-urchins, and sea-cucumbers, animals 

 mostly sluggish and calcareous, deserving their title of 

 thorny-skinned or Echinodermata. Here again, moreover, 

 the sea-cucumbers or Holothurians exhibit features which 

 suggest that this class also originated from among 

 " worms." 



But " Worms " form a vast heterogeneous "mob," heart- 

 breaking to those who love order. No zoologist ever speaks 

 of them now as a " class " ; the title includes many classes, 

 bristly sea-worms and the familiar earthworms, smooth 

 suctorial leeches, ribbon-worms or Nemerteans, round hair- 

 worms or Nematodes, flat tapeworrns and flukes, and many 



