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National Geographic : 1967 Mar
Contents
moments or even days later; then the sperm fertilizes the eggs. During a ten-day vigil at Woods Hole Marine Biological Laboratory in Massachusetts, veteran staff man Robert F. Sisson captured the unusual photographs of the breeding cycle of Loligo on these and the following four pages. for his classic anatomy of the Mollusca. Dried in blocks, the ink can be employed as a water color pigment. Its potency is long-lasting; good-quality writing ink has been prepared from the ink sacs of fossil cephalopods more than a million years old. The quantity of ink yielded by a single animal is amazing. A few years ago, I ob tained a freshly stranded blanket octopus. Slipping it into a three-gallon jar of alcohol, I accidentally broke the ink sac, and its con tents began to seep out. In moments the entire jar was black. Quickly, I removed the octopus and washed it thoroughly for about twenty minutes. With everything on and around me black, I put the animal into a fresh jar of alcohol-which in no time also became black. Squids usually cruise slowly through the water, propelled by fins along the sides of their mantles. But when they want to go somewhere in a hurry, they do what we do-they go by jet. Drawing water into the body chamber, or mantle cavity, they squirt it out through a tapered funnel, which can be turned forward, backward, or even sideways (painting, pages 392-3). A valve even prevents backfire! While diving on the Florida reefs, I often watch squids of the species Sepioteuthis sepi oidea (page 410). They seem fearless and ap proach to within a foot or two, hovering in the water and rippling their narrow marginal fins just enough to hold themselves motionless. But if I reach out my hand to the squid, a flash of color passes over it, its mantle contracts, and it shoots away. The creatures are curious, though, and usually return within a short time, watching my every movement. Much of the squid's speed originates in an unusual set of nerves in the upper surface of the mantle. These nerves, containing a system of giant fibers, are so highly developed that when the squid is threatened, a nerve impulse flashes to all parts of the mantle simultane ously, the muscles quickly contract in unison, and the animal jets safely away. Giant Nerves Aid Medical Research Scientists first began to study these nerve structures in the 1930's. And then came the big surprise. In some squids, such as Loligo pealei, common in the western Atlantic, and Dosidicus gigas of the eastern Pacific (pages 394-5), these fibers are tremendous-as much as 1/10 of an inch in diameter, as against 1/1000 of an inch for the largest in man. This is the most massive inner signal system known in the animal kingdom, and consequently the most easily studied. Since that time, researchers have flocked to squid laboratories in North and South Amer ica and in Europe to work on squid nerves. Much present knowledge of the actions of hu man nerves, their physiology, biochemistry, and physics, we owe to the squid. No one really knows how fast squids can swim, because their movements are so erratic. They are certainly among the swiftest animals in the oceans. Some can even shoot thirty or forty feet out of the water, gliding over the waves for more than a hundred feet. Not in frequently, they land on ships' decks. Equipped with flashing lights, ink, and in credible speed, squids are well adapted to life in the sea, but these fantastic creatures seem especially blessed with survival mechanisms. One is an ability to change color so fast that 399
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