Science: No Backbones

In the competitive human world, a common platitude says that a man needs “backbone” to succeed. In the competitive animal world it is different. Scientists have other criteria than fame, money and power for measuring biological achievement. If they were polled they would probably award the gold medal of greatest biological success to the arthropods, a phylum (subkingdom) of invertebrates which includes crayfish, shrimps, lobsters, crabs, water fleas, barnacles, spiders, scorpions, ticks, insects. Reason: The phylum of arthropods (the name means “jointed legs”) has the greatest number of species and individuals, occupies the widest stretches of territory and the greatest variety of habitat, consumes the largest amounts and the most diverse kinds of food, defends itself most capably from its enemies. Of more than a million known species of animals, 95% are invertebrates, and over three-fourths of them are arthropods.

This is one of the points made in Animals Without Backbones, a lively, copiously illustrated survey of the invertebrate world published recently by Zoologist Ralph Buchsbaum of the University of Chicago.* Other highlights:

>The lowly, cross-eyed little flatworms called planarias are highly regarded by zoologists, for they are the “lowest” animals in evolutionary rank which possess a definite head, right and left side, top and bottom, front and rear. Though it has no backbone, the flatworm is tenacious of life. If it is decapitated, the head will grow a new body, the body a new head. If the head is divided by a longitudinal cut toward the tail, each part will become a complete new head, making a two-headed monster. If part of a flatworm’s head is removed and grafted to a wound in another flatworm’s body, a new head will indomitably sprout from the wound (see cut).

>Largest of living invertebrates are the giant squids. In the early 19th Century a sea captain brought on board his ship fragments of such a giant which showed it to have been 50 ft. long, not including the ten arms. Scars of combat with giant squids have been found on the hides of whales. Largest of known insects, extinct for 170,000,000 years, had a wingspread of 2 ft. 6 in. Largest of known arthropods was Pterygotus, 9 ft. long, which faintly resembled a lobster and roamed on the Silurian sea bottoms of 350,000,000 years ago.

>Clams move by pushing their fleshy protuberance or “foot” into the sand, swelling out the tip so that it acts as an anchor, pulling itself after, repeating the process. Fresh-water hydras (primitive digesting stalks with predatory tentacles at the top) sometimes move by “somersaulting”—bending over, attaching their tentacles to the bottom, flipping over, and so on.

>The compound eyes of the dragonfly, which occupy most of its head, have nearly 30,000 units.

>Some sea cucumbers, when attacked, eject their viscera as a tempting distraction. If the ruse is successful and they escape, they can easily regenerate new insides.

* University of Chicago Press ($5).