LESSON XVI.

THE FIRST CRUSTACEANS.

“Look on this beautiful world, and read the truthIn her fair page: see every season bringsNew change to her of everlasting youth;Still the green soil with joyous living thingsSwarms, the wide air is full of joyous wings,And myriads still, as happy, in the sleepOf ocean’s azure gulf, and where he flingsThe restless surge.”

“Look on this beautiful world, and read the truth

In her fair page: see every season brings

New change to her of everlasting youth;

Still the green soil with joyous living things

Swarms, the wide air is full of joyous wings,

And myriads still, as happy, in the sleep

Of ocean’s azure gulf, and where he flings

The restless surge.”

—Bryant.

In those long ages when there were no human beings in the world and the lower orders of animals had the globe all to themselves, each successive period of change was distinguished by some special class of living creatures, which were then chief in numbers and importance. Each of these inits own age reached its prime in size and dominion. But while these particular families passed away at the close of their earth age, the classes to which they belonged frequently survived in changed forms, which have continued to the present. These late descendants of the old time possessors of the world have generally decreased in size and numbers, and increased in beauty and in capacity for ministering in some way to humanity.

CHILDREN OF THE DAWN.

But there are some ancient classes of living creatures which have changed very little, and are to-day practically what they were at their origin. Thus the foraminifera were among the very earliest of living creatures, as they were also among the smallest, the most simple in structure, and the most widely distributed. Their work seemed to be first tocollect material from sea-water to build into their own forms, and then to bequeath themselves to the formation of rock. There is no family of creatures so ancient, for beside them all other fossils seem modern. As living animals they still survive in almost unchanged conditions, and abound everywhere except in the Polar seas. In the tropics they are found in shallower waters than in the temperate zones.

The hard name of these creatures comes from the wordforamen, a little hole like the eye of a needle. Now these queer animals were made of a soft substance like jelly or glue in shells with many divisions. The walls of the lime-cells or rooms were full of tiny holes like the holes in a sieve, and through these the jelly part of the animal poured itself, and so filled and pervaded all the chambers or cells. Many of these creatures can be seen only through a microscope. In a former lesson we spoke of the nummulites, large specimens of this same order, and of their work in stone-making. We found that most of the building-stone of Paris, vast beds of stone in India, and a large part of the chalk rocks are nummulitic. The great pyramids of Egypt and the enormous terraces upon which they stand are built of the same nummulitic limestone, while many other vast ledges of limestone rock are composed of the minute shells of foraminifera.

In most ancient times the coral polyps were building up their towers, and were perhaps larger and more plentiful in the lower Silurian age than at any other period; but coral polyps are still busy on the Pacific sea building their beautiful islands or atolls.

The mollusks, or shell-fish, with single or bivalve shells, with heads or without,[32]were, no doubt, larger and more abundant in the Silurian epoch than ever after; but they remain plentiful now, as all collectors of shells are glad to realize, and though in many orders they have decreased in size, they have increased in beauty of form and coloring.

The crustaceans, or crab family, were among the earliest of living creatures. The class survives in many varieties, but some of its earliest families have entirely perished. In the age in which they first appeared, the crustaceans were the most numerous of living things. The crustaceans are of the ringed class of animals, to which also insects belong,[33]and here we find that the ringed creatures of the sea preceded by a long time the ringed creatures of the air and land; for crabs of many kinds crawled in the shallow seas and were cast upon the rocky shores of the earliest continents and islands long before a solitary dragon-fly or cockroach found life, or could have found a wooded or grassy piece of land upon which to rest.

The first of the ancient crustaceans which we shall mention is represented by a very numerous class of fossils called trilobites. There are no living trilobites, but the creatures now nearest them are the isopods. The trilobite was more like an isopod in its larva state than like a full-grown one. The trilobite had a hard plate or shell over his head and the front part of his body. This piece of armor was shaped nearly like the large front portion of the shell of a horseshoeor king-crab. The king-crab uses this shell as a mud-plough to plough its way through the slime, where it finds its food and deposits its eggs.

Some of the ancient trilobites were as small as a split pea, and some were a foot across their head-plate; but large or small they all had the head-shell and used it for the same purposes not only for protection, but as a mud-plough for digging burrows in the ooze below the shallow seas or on the shores. On the sides of this hard buckler were placed the eyes, which were large, prominent, and compound, or furnished with many lenses, as we find in the eyes of insects.[34]These round, prominent eyes are a very marked feature of a fossil trilobite; they seem to be staring at creation still, though the creature has been dead thousands of years.

The body of the trilobite was composed of numerous rings or segments, and each of these rings was divided into three lobes, from which it receives its name, tri-lobe-ite, or three-lobed animal. All the rings of the body were so joined that the trilobite could roll itself into a ball, doubling its tail forward under its head, and so bringing all parts of its body under the protection of the horny shield. Have you never, in turning over earth or breaking a decayed stump, found numbers of small, oval, soft, gray, ridged creatures called slaters or wood-lice? Have you noticed how as soon as they are disturbed they roll themselves into an apparently lifeless coil? In this fashion some of the trilobites were like the wood-louse.

For a long time it was supposed that the trilobite had no legs; but at last it was found that they had a number of small, thin-jointed legs, like those of a young spider-crab. Thus we see that in its head-shield and eye-placement, the trilobite was like a horseshoe crab, or limulus, which is no doubt its near relation; in general form it was like a young isopod; in its method of rolling itself up for rest or safety, it was like a wood-louse; in its long, thin legs it was like a small spider-crab.

During one earth-building age, the trilobite was the chief representative of the crabs, and many hundred species of trilobites have been discovered.

We find that their patterns varied much in the days when they were the leading order of the earth. The track of the trilobite in the mud in which it lived was peculiar, and has gained the name of “ladder footprints.” These footprints have been preserved in the hardened mud, as have rain-drop prints and birds’ tracks. The slim, sharp-toed feet of the trilobite made little rows of dots, and on each side of these was a deep groove, the rows of dots looking like the rounds, and the grooves like the sides of a ladder. This double groove was caused by the dragging edges of the shield or head-plate, and the ladder footprint is much like the track of a king-crab.

Floating above the primordial mud, in good fellowship no doubt with the trilobite, was the brachiopod, or “arm-footed” creature. This name was given because of numerous slim finger-like appendages, such as the barnacle uses for its fishing apparatus.[35]The bra-chi-o-pod was partly like the crustaceansand partly like the mollusks, while some learned men have concluded it to be closely related to worms and star-fish! In fact these ancient families of animals had many points in common. Perhaps the ancient lamp shells were the representatives of the brachiopods when all these first families of the world lived together in the Cambrian seas.

The brachiopods had two valved shells of very lovely shapes. They were a prophecy of the wonderful beauty that was to appear in the world. Some of the shells were only a quarter of an inch across, others were four inches, and a few varieties have been found with shells a foot wide. These were giants of their class. The outer surface of these shells was exquisitely chiselled, as a jeweller ornaments gold and silver plate, and the beauty of the shell was increased by brilliant colors. Red, green, yellow, blue-black, and rose-pink were the hues in which these pretty creatures disported themselves before the big staring eyes of the trilobites. Many of these animals moored themselves by peduncles to rocks or to other creatures, as barnacles do, and thus holding fast, they fished for their dinners with their fine net of filaments; for these delicate threads were not for locomotion, but were mouth appendages to catch something to satisfy the creature’s hunger.

The brachiopods were, next to the trilobites, the most numerous and important group in the Cambrian age. But, while the trilobites died out with their age, some of the representatives of their pretty neighbors have existed ever since, and, no doubt, in the Silurian days they lived as contentedly among the corals, as they now live among coralgroves in the South Pacific. Observation of a living animal of this class shows that having fastened itself by its peduncle to some firm holding place, it can very rapidly fold and unfold its finger-like filaments, creating around itself a little whirlpool, which inclines towards its mouth, and draws its food within reach.

FOOTNOTES:[32]See Nature Reader, No. 1, Lessons 34-36.[33]Nature Reader, No. 2, Lesson 1.[34]Nature Reader, No. 2, Lesson 17.[35]Nature Reader, No. 2, Lesson 34.

[32]See Nature Reader, No. 1, Lessons 34-36.

[33]Nature Reader, No. 2, Lesson 1.

[34]Nature Reader, No. 2, Lesson 17.

[35]Nature Reader, No. 2, Lesson 34.

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