3 ELEMENTARY ZOOLOGY 



ficial details of position, appearance and general structure 

 markedly different. Animals are fitted to live in different 

 places amid different surroundings by having their bodies 

 modified and the performance of their life-processes modi- 

 fied to suit the special conditions of their life. 



Vertebrate and invertebrate. In selecting the toad 

 and the crayfish as the first animals to study and to com- 

 pare with each other, we have chosen representatives of 

 the two great groups into which the complexly organized 

 animals are divided, viz., the group of backboned or 

 vertebrate animals, and the group of backboneless or 

 invertebrate animals. To the vertebrates belong all those 

 which have an internal bony skeleton (and a few without 

 such a skeleton) and which have also an arrangement of 

 body-organs on the general plan of the toad's body. A 

 conspicuous feature of this arrangement is the situation of 

 the spinal cord or main great nerve-trunk along the back 

 or dorsal wall of the animal, and inside of a backbone. 

 All the fishes, batrachians (frogs, toads, salamanders, 

 etc.), reptiles (snakes, lizards, alligators, etc.), birds, and 

 mammals (quadrupeds, whales, seals, etc.) belong to the 

 vertebrates. / The backboneless or invertebrate animals 

 have no internal bony skeleton and have their main nerve- 

 trunk usually along the ventral wall of the body, some- 

 times in a circle around the mouth, but never in a back- 

 bone. To the invertebrates belong all insects, lobsters, 

 crabs, clams, squids, snails, worms, starfishes and sea- 

 urchins, corals and sponges, altogether a great host of 

 animals, mostly small. 



7 



