The Lace Weavers

For 300,000,000 years tiny animals have been weaving delicate lace. They weave constantly, rapidly and in lovely, open mesh patterns. They make a stiff stable lace. Their own limestone entombed bodies are the threads. Night and day, millenium after millenium, they weave and weave, for the curse of weaving is forever upon them. Through time they have covered hundreds of square miles with white and green veils. For the most part these are fragile and short-lived, but in a few cases they have been preserved untorn through the ages.

These lace weavers are the bryozoa, or moss animalcules—one of the oldest, most abundant and least known forms of animal life. They have much the same habits as the corals, but the two limestone secreting creatures are not even remotely related. The weavers are far higher in the scale of evolution than the island builders. Their family associations long have been in dispute. They have been associated with the rotifers and mollusks and even with some unknown ancestral form leading to the vertebrates. Now, however, it is believed that their nearest relatives are the nearly extinct brachiopods, or lampshells.

The two groups started at about the same time in the Cambrian geological period of half a billion years ago, but they followed different paths of development. Both might be considered proto-mollusks—very remotely kin to clams and oysters. For milleniums the brachiopods flourished in the primaeval seas. During the Permean period, about 300,000,000 years ago, they constituted one of the most abundant forms of animal life. Now they seem close to the end of the road. The weavers are as flourishing, and busy, as ever.

Like a coral polyp or the larva of a clam, the bryozoan starts life as an almost invisibly minute, free-swimming creature, usually less than a thirtieth of an inch long. After a few weeks it settles on some hard surface, usually a stone, and secretes its limestone shell. New individuals rise from the body of the founder of the colony at various angles, depending on the particular design of the tapestry being produced. Each of the buds, after achieving its coat, sends out new buds. This is the weaving process.

The outside of the stone coat often is marked with delicate and bizarre designs discernable under a microscope. These designs always are the same for members of a colony and quite similar for an entire species. They make it possible to identify species in geological formations and this eventually may be of considerable importance for oil geologists. After death a colony usually is broken up quickly by wave action. Sea bottom ooze often is filled with the remains. This ooze, over periods of milleniums, becomes compacted into rock.

The weaving process may be very rapid. A colony, starting with a single free-swimming larva, may cover as much as 100 sq. feet. Such colonies have been found on a single stone. They often are found on mollusk shells. At present the bryozoans are economically important chiefly as a menace to the oyster industry. Once they have covered an abandoned shell, oystermen believe, no other oyster will make use of it. About their only other importance to man comes from the fact that some fresh-water species may clog water pipes by their rapid growth.

Every bryozoan in a colony remains throughout its life a separate animal, shut off from its fellows by a wall of limestone and leading an independent existence. Nevertheless, in the species pattern it assumes, each colony acts as if it were a single organism.

Moreover, a phenomenon unique in nature, every individual appears to be two and in some species three animals in one. Each leads its own life and dies its own death at its own time. But all make up a single microscopic whole.

First is the zooecium, a limestone-encrusted box of tissue. This is the continuing individual. Inside the box is a little tentacled worm, the polypide. It contains all the vital organs—the brain and the nerve, circulatory and digestive systems. It breathes, hunts, eats and lives quite independently of the zooecium. This polypide usually is short-lived. It has no excretory system. Poisons pile up. It degenerates and dies. When it expires the cells of the zooecium wall assert themselves. From the dead cells of the polypide they extract what nutritive material is present. The “inside animal” becomes a brown speck-like body. Then the zooecium cells sprout a bud which becomes a new polypide. This lives its normal life span and suffers the same fate as its predecessor. Another brown body is the only evidence that it has lived. This process may be repeated ten or twelve times. Think of a man, or any other high animal, which could replace over and over again its entire internal system with another made out of its own skin which had eaten its own defunct brain and heart.

The relation of zooecium and polypide as it exists in one type of bryozoa, the so-called “sea mats”, was vividly described by the great British naturalist P. H. Gosse. These are not lace weavers. They form a colony which looks like a pale, yellow leaf, such as Gosse found in a microscopic study of a mass of sea weed in which he saw other animals like “exquisitely crimson leaves thinner than the thinnest tissue paper, with tall and elegant dark red feathers and purple filaments each as fine as a silk worm’s thread.”

“Each individual cell [zooecium] of the sea mat”, Gosse tell us, “is shaped like a child’s cradle. Suppose a coverlet of transparent skin were stretched over each cradle, leaving an opening just over the pillow. Suppose in every cradle there lies a baby with its little knees bent up to the chin in that zig-zag fashion in which children often lie.

“But—the child is moving. A slowly pushed open semi-circular slit of the coverlet and we see him gradually protruding his head and shoulders in an erect position, straightening his knees at the same time. He is raised half out of bed. His head bursts open and becomes a bell of tentacles. This baby is the tenant polypide.

“The chambers themselves show signs of life. Their front doors suddenly open, gape widely and shut with a snap. This opening and shutting is repeated over and over again. The polypide emerges from the cell slowly and withdraws like lightning at the slightest alarm.”

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.