PROBLEM 



1 What Kinds of Animals Inhabit the Earth? 



The animal kingdom. We often speak 

 of two large groups of animals: the 

 v^ertebrates, animals with a backbone; 

 and the invertebrates, animals without a 

 backbone. A backbone consists of sep- 

 arate little bones {vertebrae — ver'te- 

 bree). The vertebrate group is very 

 large and is subdivided into five classes: 

 the mammals, the birds, the reptiles 

 (snakes and their relatives), the am- 

 phibians (frogs and their relatives), and 

 the fish. All these animals, diff^erent as 

 they may seem at first glance, have im- 

 portant resemblances. Besides the back- 

 bone, they all have a brain in a boxlike 

 skull {craniinn). Attached to the brain 

 is a spinal cord. It lies along the animal's 

 back, protected by the backbone. All 

 animals having these characteristics are 

 called vertebrates. 



The vertebrates, together with some 

 other less familiar animals, are called 

 chordates (core'dates). We shall not refer 

 again to the other chordates. The name 

 phyhmi (fy'lum) is given to such a big 

 grouping as the chordates. 



The invertebrates are arranged in 

 many groups or phyla (fy'la). There are 

 many more kinds of invertebrates than 

 vertebrates. And the number of individ- 

 uals is much larger, too. Commonly 

 known invertebrates are the insects, the 

 spiders, the lobsters, the clams, the snails, 

 the starfish, the worms, the jellyfishes, 

 the corals, the sponges, and the mi- 

 croscopic animals known as protozoa 



(proe-toe-zoe'ah). All these belong to 

 the animal kingdom. So the ants which 

 are insects have as much right to be 

 called animals as dogs or horses or birds. 

 All belong to the animal kingdom. 



Subdividing the animal kingdom. You 

 read that the animal kingdom is divided 

 into large groups called phyla. A phylum 

 may be divided into subphyla; generally 

 it is divided into classes. Now this book 

 and many other textbooks are divided 

 into units and the units are subdivided 

 into problems and the problems into para- 

 graphs. On more or less the same prin- 

 ciple a phylum is divided into classes and 

 the class is divided into orders. In a later 

 problem you will see that the subdivid- 

 ing does not stop there; it goes right on 

 until you have the followmg: 



Phylum 

 Class 

 Order 

 Family 

 Genus 

 Species 



The word species (spee'shees) means 

 kind of animal (or plant) such as the 

 dog species or cat species, the lion species, 

 the horse species, and so on. Sometimes 

 the species is subdivided even further 

 into varieties or breeds. 



In reading about animals in this prob- 

 lem you will concern yourselves mostly 

 with phyla and classes and some of the 

 species of animals they include. 



