To their activities as scavengers of the forest floor these ancient worms owe their place in the economy of nature. They normally feed several times a week. When kept without food, however, they can stay alive for months. They gradually shrink in size as they digest themselves. The internal organs are reduced little by little as they are absorbed for food. The first to disappear are the reproductive organs. Most planarians have both male and female reproductive systems. Then come the muscles of the body wall. Never however, do the worms eat their own brains or nervous systems, although the brain may be reduced greatly in size. The I Am of the worm can devour its vestments of protoplasm; it cannot eat itself. When food is available again the organs are regenerated and return to normal size. Instances are recorded where planarians have reduced their length from slightly more than an inch to less than a seventh of an inch in six months.
Closely related to this practice of “eating themselves” is the remarkable ability of the terricolae to break themselves into small fragments each of which will regenerate into a complete worm. This capacity probably has been a major factor in their survival through the aeons since multi-celled life began on earth. What might seem to be their outstanding weakness in the constant struggle for survival—their soft bodies and extremely loose organization—has become their major strength. A planarian can lose at least nine-tenths of its body and still preserve its individual existence. This self-shattering phenomenon constitutes the worm’s chief defense in emergencies. It comes into play when any danger threatens. The regenerating ability, especially of fresh-water forms, differs considerably in degree from species to species. Some are unable to regenerate a “brain” out of fragments of the rear part of the body. Complete in every other respect, the remade worms seem incapable of the typical gliding movements of the race. They remain quiet most of the time but can move forward slowly. A tendency to move in circles has been observed. Fragments from the head section, however, quickly become complete animals.
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.