The science of life : fully illustrated in tone and line and including many diagrams

LESS INDIVIDUALIZED ANIMALS

spread tentacles stir restlessly, and occasionally, when the creature is alarmed, it ducks back, contracted, for a few seconds into the safety of its horny cup, but it never wanders or explores, nor does it reproduce its kind ; it leads a life of patient waiting and grabbing and digesting.

Compared with any of the animals with which we have dealt hitherto, the anatomy of the polyp is incredibly simple. It consists of two layers of tissue, one inside the other and separated by a thin gelatinous sheet of non-living substance, like the firm matrix of our gristle. It has some seven kinds of cell, as opposed to the seventy or so that are found in our bodies. It has no heart, no eyes, no ears ; as far as we know, it has only two senses, taste and touch ; it has no brain, its nervous system being scattered through its body and organized about as elaborately as the local nervous system that produces writhing movements in our own bowels. Except for the mouth, tentacles, and gut of which it consists, it has no specialized organs of any kind.

At its base the polyp tails away into a narrow tube of living tissue, running down the branch that the polyp crowns, and this leads into a main tube that forms an axis for the colony. Every polyp on the little frond is directly continuous with this central tube of tissue, and, through it, with every other. They are like flowers growing from a common stem, one flesh. Occasionally a side-branch grows out from this main tube, making a clear, horny sheath as it goes, and gives rise to a new polyp. Thus, by a process of budding and branching, the colony grows.

But occasionally, here and there along the central stem, there arise polyps of a new kind. Buds sprout out, at first very much like the usual kind of bud, but later developing into plain, club-shaped cylinders called blastostyles. The blastostyles do not develop mouths or tentacles; they are not feeders but breeders. They become covered with tiny wart-like buds which grow and develop into little free-swimming creatures. There is a division of labour between the different parts of the colony; the cavities of the stomachs of the feeding-polyps are in direct communication with the bore of the tubular stem, and some of the products of their digestive processes diffuse along this channel to the other branches. The feeding-polyps nourish the whole colony. The blastostyles, like our own ovaries or testes, do not contribute to the economy of the colony to which they belong, but, nourished by the

diffusing juices from the feeding-polyps, they concern themselves with the founding of fresh colonies. The feeding-polyps can move independently, and if they are cut off from the stem the material of which they consist lives on and reorganizes itself into a new colony ; nevertheless, biologically speaking, they are parts, not wholes.

The buds that are produced in great

Fig. 99. Part of a colony of Obelia, seen through the microscope. Above are three polyps in various stages of expansion. Below are a Blastostyle on the right and a free-swimming Medusa on the left.

numbers by the blastostyles develop into miniature jelly-fish, as clear as glass, and little more than a tenth of an inch across. Their shape is that of an umbrella with a very short, thick handle ; they have a hemispherical bell fringed with tentacles, and, hanging from the middle of it, a short stalk which bears the mouth at its end. As they become properly developed they detach themselves from the blastostyles and swim actively and independently through the

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