What Are Animals?

Concept Note

We usually think "mammal" when we hear the word "animal." In fact, we and this Rhinoceros beetle are animals.

In the Animal Kingdom, a small swimming sperm makes it to a large egg, spurred on by its undulating tail. The fertilized egg repeatedly divides to form, in the initial stage of embryo development, a hollow sphere of cells - the animal blastula. This blastula is the defining trait of animal-hood.

The bodies of animals are individualized with special cell-to-cell connections. As the embryo cells divide, some must form alliances, while most others die on a preprogrammed cue, If these cells do not commit cellicide in the proper fashion, no animal body develops.

The following true, or untrue, bug story cites J.B.S. Haldane, hero of evolutionary biology. At a formal dinner, Haldane was seated next to his staunch foe, the Archbishop of Canterbury. There was, of course, "polite" British exchange:

Archbishop: What do your studies tell you, Professor, about the nature of the Creator?

Haldane: He must have had an inordinate fondness for beetles.

Insects innovate more successfully than do any other animals. Close to a million different species have been identified ... and we're still counting.

Clockwise from Top: Beetle sperm penetrate egg, which divides to evolve sequentially into blastula, larva, and beetle. (Illustration by Christie Lyons)