All planarians actually have heads and a “brain,” of sorts. The latter consists of two minute bits of nerve tissue just behind the front of the body, oval-shaped and enclosed in a tough capsule. It serves as a center for nerve fibers extending throughout the animal. Here are coordinated the stimuli received from light and heat, and possibly those from odors and sound. When the worm goes forward, it moves its head constantly from side to side. Presumably it is exploring the way ahead for food and danger. A terrestrial flat worm’s progress is described as “gliding,” rather than creeping or crawling. The outer surface of the body has many glands from which is exuded a mucus over which it slides. This mucus quickly hardens. From it can be made slender threads by which the worm, like a spider, can lower itself safely from projections. Because of the glue-like quality of the secretion it is able to climb perpendicular surfaces. From the hardened mucous, sometimes mixed with sand, it can make for itself a shell into which it can retire for months at a time. _Queer Fish, But Definitely_ There are more than 40,000 kinds of fish in the world. Their habitats range from the profoundest depths of the seas to cold lakes and brooks on mountain timberlines. They show a bewildering diversity in their ways of life. The smallest of fish is a Philippine goby, less than a third of an inch long and weighing a fraction of an ounce. The largest is the whale shark, found in all warm seas. Some individuals exceed twenty tons. Some fish burrow in the mud, some swim, some walk, some fly, some breathe air. Some are timid, some bold and bloodthirsty. Some are placid, some easily irritated. Some are highly venomous. One, found in Australian waters, weighs nearly half a ton and has poison barbs a foot long. Some of the deadliest are among the most beautifully colored. Freshwater fish can sometimes be cut out of cakes of ice in which they have been frozen for months at a time, and completely revive. Actually the fish themselves are not frozen. The freezing point of their blood is slightly lower than that of water. They were merely “hibernating”. This may happen frequently in nature. Some fish seem well on their way to becoming land animals. They can breathe in air better than in water. Surgeon fish are so-called because of a sharp spine on the tail which can produce a cut like that made by a surgeon’s scalpel. Parrot fish have beaks like parrots with which they scour algae from the coral reefs for food. Goat fish have two growths under the mouth which look like the chin whiskers of goats.