i THE WEALTH OF LIFE 9 



become so degenerate that no zoologist ignorant of their 

 life-history could recognise their true position. Below 

 this come certain claimants for Vertebrate distinction, 

 notably the Enteropneusts, such as Balanoglossus, which 

 serve to link the backboneless and the backboned to- 

 gether. They are either worm-like vertebrates or verte- 

 brate-like worms. 



Across the line, among the backboneless animals, it is 

 more difficult to distinguish successive grades of higher 

 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, (a) the cuttle- 

 fishes 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 



*/ mr Ji 



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 exclu- 

 sively aerial or terrestrial spiders and scorpions, insects 

 and centipedes, besides quaint allies like the " king- 

 crab," the last of a strong race. Again a connecting class 

 demands special notice, the Onychophora, such as 

 Peripatus, old-fashioned, widely represented terrestrial 

 animals of caterpillar-like or worm-like appearance. 

 They breathe by air-tubes, somewhat like those of in- 

 sects ; they get rid of their nitrogenous waste-products 

 by means of kidney-tubes somewhat like those of certain 

 worms. 



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 thornv-skinned or Echinoderma. 



