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

BOOK e

special adhesive organs. By means of these tentacles the creature catches its preysmall fishes, crustaceans and the like—and pulls them into its mouth, which is on the lower surface of the globe.

Most Ctenophores live, like the seagooseberry, at the surface of the sea ; most are transparent, tinged with pink or yellow, and some are brilliantly phosphorescent. The wonderful ‘“‘ Venus’ Girdle”? of the Mediterranean is a rippling, band-shaped animal, delicate violet in colour, with a brilliant greenish fluorescence, and may attain a length of five feet. One or two exceptional members of the group do not float but creep over the sea-bottom.

Ctenophores develop directly from the egg without any intermediate polyp-stage.

8 4 Sponges

It has probably never occurred to many of us that we get into our baths in company with askeleton. The sponge that the reader employs—unless, of course, he or she prefers the flaccid rubber substitute that is attempting to oust the natural article—may be variously formed, for there are several species on the market, but it is most likely to be a more or less rounded object, a flattened globe. Perhaps it has been trimmed a little by the dealers ; most of the sand that had to be washed out of it after it was bought had been artificially introduced, for sponges are sold by weight. There is nothing very suggestive of a living thing about the domestic sponge ; and yet when it is examined with care it reveals some degree of organization. ‘The caverns that riddle it are of various kinds ; there are conspicuous round holes, often large enough to admit the finger, and leading into neatly trimmed, more or less straight cylindrical shafts that sink deep into its substance, and there are smaller, less regular holes, leading into a labyrinth of passages some of which open into the main shafts. Moreover, the substance of the sponge is itself a network of branching fibres, a network so fine that its meshes are only just visible to the naked eye. These fibres are the picked and whitened bones of a living thing.

At one time the sponge was covered with flesh, and it lived at the bottom of a warm, shallow sea—round the shores of the Mediterranean, the West Indies, Australia. It was a yellowish, brownish, or dark purplish object, smelling faintly of garlic, elastic and slightly slimy to the touch; living bath-

THE SCIENCE OF LIFE

CHAPTER 4

sponges have been compared, in colour and consistency, to fresh liver. Most of its holes were Closed with a thin skin of tissue. Some of the larger ones—called vents or osculawere open, although most of these could be closed at will by means of films of tissue round them, and scattered freely over its surface there were microscopically fine pores.

The creature did not move from place to place, it remained rooted firmly to a rock, and, except for occasional slow changes in the diameter of its oscula and pores, it made no visible movements. Indeed, sponges are such passive creatures that for a very long time their animal nature was hardly even suspected. They: grow, and therefore there must be life in them of some sort ; but many biologists considered them to be plants, and a few even believed them to be the dead by-products of living things, secreted by the worms that are invariably found crawling about in their cavities, much as the gelatinous investment of a sea-squirt is secreted, or the tubes and cups of a coral. But just over a hundred years ago Robert Grant made an observation of fundamental importance. He noticed that there was always a slow, steady current of water through a sponge—passing in through the microscopic pores, out through the vents. That current is the essential vital activity of the sponge ; its means of livelihood and almost its only means of self-expression.

Sponge-cake, spongy iron, spongy platinum —the name has come to mean anything riddled with holes, for a sponge consists in the main of a tangle of aqueducts guiding its precious stream. Its cavities are of three lands : there are the “ flagellated chambers” where the current is both caused and exploited, there are inward passages guiding the current from the surface pores to the chambers, and there are outward passages guiding it away to the oscula. A flagellated chamber is a microscopic cavity lined with living cells possessed of long whips like the tails of spermatozoa ; by lashing their whips they keep the current moving through the chamber, and indeed through the whole sponge. Round the base of its whip each cell has a transparent collar, a collar which has the power of grasping suspended organic particles as they drift by and passing them down to the cell below. They are then taken into the cell itself, and there digested. The current, then, is like the current that flows through the pharynx of a sea-squirt, or the gills of a clam; it brings food and oxygen, and washes waste matters away,