JUST beneath the surface of the sea, like a horizontal rain of rockets, dozens of pale forms streak away from the bow of a boat. They shoot into the surrounding darkness with a swirl of luminescent water in their wake. A school of squid has been startled, and this fleeting glimpse is usually the only evidence.

The squid’s soft body is sleek and smooth. Its safety depends on speed. Rapid escape is so essential that from head to tail the creature is streamlined for retreat; it is powered for fast movement backward. The propulsive force for this arrowlike flight is sea water, squirted from a single nozzle on the animal’s underside. Unlike the jet motor of a V-bomb, the squid’s rocket mechanism operates in fast-repeated pulses. And even as it hurtles backward through the water, the animal reloads. It allows its head and arms to trail behind, and in doing so exposes a slender neck. Around this neck, as though into the space left by an oversize collar, the squid sucks up water to fill a spacious chamber. When charged once more, the creature pulls back its head to stopper up the hole through which the water was taken, and contracts muscular walls to force the liquid through the open nozzle. With every pulse, another yard or so is added to the distance between itself and the object which startled it. The ten arms are used also as a support when the animal rests on the ocean floor. Then the tentacles curve downward in a graceful arc, with their tips anchored to the bottom, while the squid’s body is held horizontally or is allowed to sag until the tail touches the sand as well. On hard mud bottoms the tentacles may leave a ring of marks which show the outlines of their sides and suckers. Only very recently naturalists have recognized those squid tracks and seen in them the explanation of similar marks on ancient shales and limestones. Ancestral squid lived and died but their soft bodies left no fossils. Yet tracks of them remain, and from their symmetry scientists have concluded that the early forms stood upended on the muds of long ago, ready to dart vertically upward at the first sign of danger.

By day, sharks and large fish sometimes pursue the agile squid. For this peril the animal has another means of defense. It adds to the water expelled from its jet a black secretion which clouds the near-by ocean, providing the submarine equivalent of a dense smoke screen.

Through this the pursuer cannot see that the squid has changed direction sharply and is darting off into another quarter. The blackened water clears to dusky brown, but not before the shark or fish has lost the trail.

Until recent years a close relative of the squid — the cuttlefish was hunted actively by men in boats along the shores of the Indian Ocean. Its sacful of black secretion was valuable in commerce as “India ink" or “sepia" water color. So well known became the reddish-brown pigment prepared from the ink of the cuttlefish that everyone used the term “sepia” to describe the familiar shades. Actually “sepia” is the Greek word for cuttlefish, but the source and the product have become indistinguishably blended in English.

The squid, the cuttlefish, and the octopus are all members of a very odd group related to the snails and clams, the slugs and scallops. All of them are mollusks. But instead of carrying their shells on the outside as armor or, like the slugs, dispensing with them entirely, the squid and its kind have enclosed a greatly reduced shell and now use it as an internal stiffening structure not unlike a backbone. The remnant of shell which the cuttlefish has imbedded deep in its muscular body is sold to hang in canary cages as “cuttle-bone.” That vestige of shell retained by the squid is horny and translucent. Fishermen who use the squid as bait call its shell a “pen" from a fancied similarity in shape to the feather quills our forefathers used in writing.

In eye structure the squid and its close relatives show an even greater resemblance to the vertebrate animals. By a totally different process, they have developed a pair of camera-like organs, each with a pupil and a lens. As in our eyes, the lens in a squid can focus a good image of surroundings on a retina, and it is obvious that these animals can see as well as the fish which pursue them.

Squids have a remarkable ability to change their colors, but the alterations seem to have no close correspondence to environment. It is not camouflage, but rather the expression of a restless nervous activity. In the outer layer of the skin are great numbers of tiny color bags, all under the creature’s control. From each sac run many fingerlike extensions to interdigitate with those of other color cells. The pigment in the bags can be gathered into minute points at the cell center, leaving all the extensions transparent and the animal a creamy white. Or the color cells containing purple pigment may be relaxed and in a fraction of a second the squid blushes to a deep maroon. The sudden change in hue may not affect all parts at once: if the animal is excited in some way, waves of color wash around its body. Each area pulses pale to dark to pale again. Other sets of color cells — blue, red, and yellow —can be brought into action, their different pigments spreading into the fine extensions and altering the general appearance of the animal.

If you reach a hand into an aquarium containing a quiet squid, and gently run a finger tip along its side, a wave of creamy white records just where you press. This action is not unlike the pale patch which follows a finger drawn along freshly sunburned human skin. But the squid’s side does not merely recover its former coloration. The stroked area is excited to an independent display of color changes. These may last for a minute or more before the stripe disappears into the general twinkling of color cells characteristic of the animal at rest.

Although escape has been written so strongly into the squid’s design, the mollusk is not unduly timid. It attacks and overpowers living food such as crabs and small fish, and is able to open clams by sheer force. Each of the ten tentacles which surround the mouth is armed with a double row of strong suction cups which can be clamped to the prey. Like those of the octopus, every sucker has a powerful piston at the bottom for producing the vacuum needed, and the tentacles themselves are very flexible and muscular. Two of the arms are longer than the other eight, and are used in grasping fleeing food. They slow its progress until the rest of the squid’s armament can be brought to bear. A parrot-like beak may be extended to tear at a captured fish, or the squid can use its rasping tongue to clean the meat from a crab’s shell.

A hungry squid swims about actively, often in company with others of its kind. By twisting the nozzle of the rocket, mechanism, water can be squirted toward the rear to drive the animal ahead. Rapid progress in the forward direction depends on the same jet as does escape backward. The jet forms not only a propeller but also a steering mechanism. To change course, the spurting water is directed to one side and the head of the squid swings around without, losing momentum. Fins near the pointed tail keep the creature on an even keel or control its diving and ascent, much as do the vanes on a submarine. But when the squid stops its jet propulsion and moves slowly through the water, the flexible edges of the fins undulate to drive the animal ahead or backward, to turn it from side to side and enable it to survey its surroundings.

Squid mothers hover in this way over their brood until the young are almost ready to emerge. The small transparent eggs are laid in little fingers of gray translucent gelatine, attached by the parent to some sunken support such as a rock or strong seaweed. The many near-by masses suggest bunches of bananas, grouped as they are into “hands.” The newly hatched squid resembles the parent, but it is practically transparent except for its brown jaws, its amber-colored tongue, the dark pistons in its suction cups, its black eyes, and a few color cells. At first these color bags are scattered widely. They seem completely disproportionate on the tiny squid; yet they pulse and change the creature’s appearance much as in the large, opaque adult.

Along the coasts of temperate seas, various kinds of squid reach a length of a foot or so. In southern Europe and in the Orient they are esteemed as food. Among the foreign-born population of our large cities there is considerable demand for both squid and small octopus.

In the colder waters of the North Atlantic between Newfoundland and Iceland, giant squid are encountered occasionally. Some of these are over fifty feet long, with tentacles a foot or more in diameter and suckers as large as coffee cups. Battles between these monsters and whales have been described, and many whales and giant sharks bear scars which show where the rows of strong suckers were clamped to their sides. Some of the more authentic accounts of sea serpents may spring from glimpses of these almost unbelievable animals.