Part 6

As mentioned before, some bryozoans appear to consist of three animals in one. The third is the so-called avicularium, or bird’s head, also vividly described by Gosse: “The cells [of this particular species] are oblong-shaped, and look much like a sack of corn. Just below one of the spines that crown the summit of the cell on one of the edges is situated a small lump which bears a remarkable resemblance to the head of a bird. It has a strongly hooked beak with two well-formed mandibles, one of which is removable. You observe it deliberately opening, like the beak of a bird and then closing with a strong, sudden snap. The birds' heads are not inhabitants of the cells. They are not even integral parts of them. The cells have their own proper inhabitants, each leading its own life and each essentially formed on the same plan as that of the baby in the cradle. There is no visible connection between its and the bird’s head, which is cut off entirely from the interior of the cell. This head has a muscular system entirely its own. It seizes small animals but has no means of passing them into its mouth”.

The real function of these avicularia is unknown. They have been pictured as fierce watchdogs kept by the bryozoa for defense against approaching enemies. Gosse speculates that they may serve indirectly as hunters, seizing and killing small animals. The disintegrating bodies of their prey, attract hordes of smaller sea creatures which can be gathered up by the tentacles of the polypide.

The Ways of Crabs

Crabs that wear clothes, others that carry arms, and still others that march like regiments of soldiers are among the curiosities of Australia’s Great Barrier coral reef.

One crab forces the coral polyp to build a limestone palace for its abode. The female of this species lodges on the polyp when it is in the larval state and causes an irritation which forces the host animal to build up the walls. The resulting house is just big enough for the crab to move about in comfortably. There always is a door through which she obtains her food.

Another species merely sits on the end of a sprouting coral which, growing outward, makes a long, circular burrow for the crustacean. Through this it can move backwards and forwards at will. The forward part of its body is enclosed in a hard shell the exact color of the coral so that when the crab sits at the door of its burrow it cannot be distinguished from the coral.

Still another crab carries two sea anemones, one in each “hand”, wherever it goes. In its first few months of life it seizes these plant-flowers—living animals with stalks and petals like flowers which ordinarily are attached to rocks under the water—about the centers of the stalks. Thenceforth it moves about like a person carrying two umbrellas.

The most logical explanation of this behavior is that the anemones serve as weapons, killing or paralyzing small sea animals which come in contact with them. This species of anemone has stinging cells in its disk. These curious weapons are carried by the crab continuously and seem essential to its life. When one of them is taken away, the crustacean moves automatically to grasp it again. When a crab is killed slowly in alcohol it clings to its weapons even in its death struggles.

There are spider crabs which cut and wear clothes. They cut off pieces of living sponges and place them on their backs. These sponges become entangled in tiny hairs which protrude through the animal’s shell, and continue to grow until they protrude several inches over the back. Thin layers also cover the under part of the body and the legs. Every time a crab sheds its shell, it must make itself a new suit The practice probably is beneficial to both animals. The crab, living in a forest of sponges, looks like a sponge itself and is thus concealed from its enemies. The sponge benefits by being carried to new food sources. When the shell is shed the sponge simply attaches itself to a rock and continues to grow.

One of the most remarkable cases of commensalism in nature has been found by Dr. Melbourne Ward, Australian zoologist in a degenerate type of barnacle which makes its way through the thin shell of one of the Barrier Reef crabs. It wanders through the blood stream of the crab and finally comes to the surface where it forms a little sac for itself. Here it metamorphoses into another form and sends long, thread-like filaments into every part of its host’s body. In some respects it is like a cancer among higher animals, except that in this case the malignant growth is that of an individual animal of another species. It lives off the food eaten by the crab but never kills nor apparently seriously injures its host. The one notable effect, for which there is no adequate explanation, is that it changes a male crab into a female.

The soldier crabs are beachdwellers, about two inches long. They march across the hard sand in perfect order, as if they were under the control of leaders. No “officers”, however, have been observed. When approached, they burrow rapidly in waves, like a regiment of infantry. First the front rank disappears in the sand, followed in order by those behind. The regiment disappears completely in a very short time.

The soldier crabs can hardly be driven into the water. When Dr. Ward succeeded in pushing a few of them off the shore they were set upon by ferocious small fish which rapidly devoured them. Realization of this danger apparently is instinctive in the animals.

Some of the land-dwelling crabs of the mud flats dig very intricate burrows with labyrinthine cross and side galleries. Some species live in a communal life. Each crab has its own burrow, but from each there is a passage into a large central hall which seems to be a community gathering place. Other species are intensely individualistic. Each excavates an elaborate labyrinth in the mud, considers this its own home, and vigorously defends it.

During courtship some of these mud crabs perform dances like the courtship dances of birds. The male of one variety, after attracting a mate by his dancing, picks her up bodily in one of his nippers and carries her away. Another variety of sand crab seems to have perfected an engineering technique which still evades human skill—that of building a burrow in soft, dry sand. These burrows are about two inches in diameter. The crab is able in some mysterious fashion to compress the soft sand into a solid substance with its nippers.

In precision of instinctive behavior, Dr. Ward found, these Great Barrier crabs come quite close to the spiders, their distant relatives.

Ticks With Noses in Their Legs

Ticks, remote spider relatives, smell with their front legs. When these legs are amputated the tick shows no reaction to odors. It cannot smell blood but will feed on any sort of liquid sucked through a warm, moist membrane like the skin. Presumably such a tick in nature recognizes an animal as a proper source of food by smell, while a combination of warmth and moisture from the skin gives a stimulus for feeding.

The Fourth Realm of Life

There is a wind-tossed green-grey ocean between earth and sky. It is a sea on stilts, the world’s fourth realm of life. There are plants and animals of the land, of the water, and of the air—and there are plants and animals of the canopy of the rain forest, a thousand-mile-wide broken belt around the world. It covers several million square miles—the jungles of South America extending northward into southern Mexico, the basins of the Niger and the Congo, strips of southern India and Ceylon, much of New Guinea. Life is rather sparse in the perpetual, drenched twilight of the jungle floor. It is abundant in the treetops, the habitat of fantastic, and still largely unknown, plants, mammals, birds, snakes, toads, frogs and insects. These might be compared to the flora and fauna of an as yet unexplored continent.

Rain forest trees are, in general, tall, straight, and branchless until near their tops, 100 to 150 feet above the ground. There they send out a rich profusion of branches and foliage. This foliage is like a thick, rough, continuous green blanket held up by tall posts, like a net below trapeze performers in a circus tent. The top of the blanket is a place of intense sunshine. Light grows dimmer and dimmer as it penetrates the leaves and the branches. Finally, on the jungle floor, there is only about a fiftieth as much illumination as on the surface of the canopy.

In the canopy four or five kinds of monkeys take the place of man on earth as the most intelligent and adaptive animals. Primates from the beginnings of the race—the weird, squirrel-like animals of the North American dawn age forests fifty million years ago—have been semi-arboreal.

Most abundant in the tree-land are the pretty, playful, curiosity-driven, humanlike spider monkeys who play tag and throw sticks at each other in the lower branches. Best known, although less likely to be seen, are the big, black, Satanic-looking howlers.

Both of these species, in the long process of adapting themselves to high jungle life, have made third hands out of the ends of their tails. With these highly sensitive prehensile organs they not only clutch branches but sometimes carry out rather delicate manipulations.

Weirdest are the black-and-white striped, woolly-furred night monkeys. These little racoon-like creatures live in holes far up in the treetops. They come out only at night and are seldom seen. They have enormous eyes which shine like live coals among the leaves when the light of a flash lamp catches them.

Probably the most dangerous single animal of the canopy is the tamandua, or golden anteater. It is exclusively a treetop creature, about the size of a rabbit, with golden-yellow, soft, silky fur. It lives almost exclusively on termites which it harvests by sticking its long tongue, covered with a sticky saliva, into their nests. A progressive relative of the sloth, it remains motionless apparently for days at a time and is a slow, clumsy climber.

But woe to anything—jaguar, ocelot, big howler monkey, even man—that runs afoul of it. It strikes suddenly and fast with its long, curved scimitar-sharp claws, and always aims at the stomach which it rips open. No other creature will venture near a tamandua, except by accident. Probably it is voiceless, although natives have attributed to the sinister little anteater a peculiarly weird cry heard in the moonlit jungle. This now is believed to be the call of a bird.

Climbing rats are abundant in the jungle top. They feed, for the most part, on fruits. Here also is the abode of pigmy squirrels which cling, heads downward, to the tree trunks with their tails curled over their backs, squirrel fashion. These animals are about five inches long, including the tail whose length is about equal to that of the rest of the body. There is a tiny, climbing mouse with short, broad feet and sharp, curved claws. Bats, mostly small, fruit-eating animals, flutter about in the darkness. Probably there are few of the big dangerous vampires in the high treetops. They fare better on the blood of larger, ground-dwelling creatures such as tapir and peccary.

Rubber-Band Worms that Stretch and Stretch