8 INTRODUCTION. 



creatures than we can count. Let us stoop over a brook or a 

 rain-pool on a warm, sunny day, and again we shall find it im- 

 possible to calculate the number and variety of minute objects 

 that in diverse movements are flitting, darting, swimming, in an 

 independent existence. The more powerful our magnifying glass 

 the more bewildering are the wondrous forms of microscopic 

 creatures revealed to us. At the seaside, on the surface of the 

 waves, and in the depths of the ocean, living forms are not less 

 numerous. By this we can understand how it is that while our 

 great naturalists have arranged all known animals under — say 

 seven grand divisions, six out of the seven embrace only small, 

 boneless, and, to a great extent, microscopic beings. 



Included among the largest of these boneless creatures are our 

 familiar acquaintances of the garden and the shore, — worms, slugs, 

 spiders, beetles, butterflies, — all insects, in fact — snails, centipedes, 

 crabs, lobsters, star-fishes, jelly-fishes, sea anemones, and many 

 others that will occur to the memory as being boneless and blood- 

 less. By " bloodless," not having red blood is to be understood ; 

 and as it happens that all animals with a bony frame possess red 

 blood, Aristotle divided the animal kingdom into two great groups, 

 "those with blood and those without blood." 



But the fluid circulating through the bodies of insects, worms, 

 shell-fish, etc., though nearly colourless, answers to the blood of 

 quadrupeds, and is so spoken of by physiologists ; therefore we 

 may dismiss any reference to the fluids of the body, and retain 

 the distinction of bones and no bones — correctly speaking, Verte- 

 brate and Invertebrate animals — as the most explicit and com- 

 prehensive mode of separating the two great groups. All animals 

 with a bony skeleton, whether seal, fish, frog, or elephant, being 

 formed on a similar plan, of which the solid structure, the back- 

 bone, is the chief support ; and this backbone being composed 

 of a number of small bones — vertebra (see skeletons, pp. 26, 82) — 

 compactly jointed together, the term Vertebrate has been chosen to 

 designate them. Excepting serpents and some of their allies ver- 

 tebrate animals have four limbs, but never more than four. In the 

 seal these four limbs are paddles ; in the fish they are its two pairs 

 of principal fins, the pectoral and ventral fins ; in the bird the two 

 fore limbs become wings ; in ourselves arms ; in the frog and the 

 elephant they are four legs. We are apt to speak of four-footed 

 animals as "quadrupeds," in distinction to bipeds, but while frogs, 

 newts, and lizards have each four legs we do not associate them 

 with quadrupeds ; though Aristotle did call them " Oviparous, or egg- 

 laying quadrupeds." The great Cuvier divided the egg-producing 

 animals into birds, insects, and reptiles ; the latter including 



