speed that characterize the vertebrate way of Hfe. 

 In contrast, the invertebrate groups generally lack 

 any kind of rigid internal skeleton to which power- 

 ful muscles can be attached, and many of the groups 

 consist of small, soft-bodied, flabby animals that 

 drift, crawl, burrow, glide, or inch their way along. 

 Some, like the clams and the arthropods, do have 

 hard skeletons that support and protect the body and 

 provide a rigid surface against which muscles can 

 pull, but these are external encasing skeletons; and 

 a hard covering that must enclose the whole body 

 grows disproportionately heavy with increase in body 

 size. Many invertebrates move swiftly, but mostly in 

 bodies of very small size. 



A striking diflference between vertebrates and in- 

 vertebrates has been apparent to man at least since 

 the prehistoric time when he was a primitive nomad, 

 managing a precarious existence as a fisher, a hunter, 

 and a gatherer of seeds and fruits. We can imagine 

 him one day stalking a wolf and getting nothing for 

 all his skill and courage but a few slashing bites from 

 the sharp teeth of his big, fast, and intelligent verte- 

 brate adversary. Then, coming out of the woods to a 

 rocky seashore at low tide, he discovers that the 

 rocks are covered with a very diflferent kind of crea- 

 ture — a shelled animal that neither flees nor turns on 

 its attacker but lies quiet and defenseless within its 

 hard shell until this is split open, with a rock, to ex- 

 pose the soft, flabby, deliciously edible, bite-size in- 

 vertebrate within. Seashores in many parts of the 

 world bear witness to such scenes of long ago as this. 

 On the shores of Denmark, for example, there are 

 huge mounds that are filled mostly with the shells of 

 mussels, periwinkles, and cockles, but also contain 

 charred bones, stone tools, and other kinds of refuse 

 discarded in these prehistoric kitchen middens. 

 There is no difficulty for us today in distinguishing 

 invertebrate remains from vertebrate, for no sub- 

 stance quite like bone is found in any invertebrate 

 group (though they do sometimes have cartilage- 

 like materials). The texture and detailed structure 

 of bone is unique to vertebrates; and even the de- 

 posited salts, of calcium phosphate, are seldom 

 found in invertebrate skeletons, which are typically 

 of calcium carbonate. 



In radially symmetrical invertebrates there is no 

 head, and the central nervous system is a ring of tis- 

 sue encircling the animal. But in the much more nu- 

 merous bilateral invertebrates the central nervous 

 sv^tem is a pair of solid nerve cords that run along 

 the midline of the belly (not the back, as in verte- 

 brates). Each cord has swellings, the nerve ganglia, 

 that are concentrations of nerve cells and that act as 

 nerve centers. In those invertebrates that have heads, 

 the largest ganglion is in the head, where the sense 

 organs are concentrated, and it is called the brain. 

 The small invertebrate brain has room for few cells 



that are free to do much except coordinate the mus- 

 cles and relay information from sense organs to mus- 

 cles. Even if the tiny brain were capable of handling 

 much learning, there would scarcely be time for such 

 a luxury in the great majority of invertebrates, for 

 most have brief life cycles. They usually feed, grow, 

 reproduce, and die within a few weeks or, at most, 

 months. To do this, they must come into the world 

 equipped with instinctive behavior patterns, and 

 these are promptly elicited by the stimuli of their en- 

 vironment. Only to a very limited extent can they 

 take advantage of the adaptive possibilities and of 

 the flexibility of learned behavior. Though we can 

 demonstrate, even in the one-celled protozoans, 

 some capacity to modify behavior as a result of ex- 

 perience, it is instinct, not learning, that dominates 

 behavior in the invertebrate world. This is true even 

 in the generally highly developed line of evolution 

 that led to the insects. 



All invertebrates are cold-blooded; that is, they 

 have no mechanism for controlling their internal 

 body temperature, which in turn controls the rate at 

 which bodily activities can take place. At all seasons 

 they must adjust to the temperature of the external 

 environment — living actively when temperatures are 

 moderately high, becoming dormant or dying when 

 temperatures are very low or very high. None has 

 the capacity to be up and about at either of the tem- 

 perature extremes to which a warm-blooded verte- 

 brate like man can adjust. This does distinguish 

 them from the warm-blooded birds and mammals — 

 but not from the lower classes of vertebrates, the 

 fishes, amphibians, and reptiles, which also are cold- 

 blooded. In a desert at high noon or on an arctic 

 tundra in the dead of winter, we would see none but 

 birds and mammals on the move. Only in tropical re- 

 gions, where cold-blooded animals can remain ac- 

 tive at all seasons, or in all the great seas of the 

 world, where the water masses themselves act as 

 thermal regulators for the animals that live in them, 

 is it readily evident that ours is indeed an inverte- 

 brate world. 



One may seriously question whether it is logical 

 to divide the animal kingdom into animals with and 

 animals without backbones, since there are only 

 some 55,000 species of vertebrates and nearly a mil- 

 lion known species of invertebrates — perhaps several 

 million when zoologists have finally named and de- 

 scribed all of them. The vertebrates are admittedly 

 a highly successful group; and many of them, such as 

 man, are big, cunning, aggressive, and noisy, and at- 

 tract an undue amount of attention to themselves. 

 From the viewpoint of a zoologist, though, the five 

 kinds of vertebrates — fishes, amphibians, reptiles, 

 birds, and mammals — are all so similar that they 

 must be considered only as five of the classes of one 

 major phylum or group, the phylum Chordata. Shar- 



14] 



