the minutest creatures; and, as shown already, it is of these skeletons that vast deposits are composed.

Still more marked advance towards unlikeness in parts is manifest in the Infusoria, so called because readily developed in infusions of exposed vegetable matter, where they crowd by myriads in the space of a water-drop. Instead of the pseudopods of the moneron and the amoeba, we find vibrating filaments or cilia, by which supplies are swept into the body, which is furnished. with a rudimentary mouth and short gullet, through which the food and oxygen pass to the body-cavity.

2. Cælenterata.

The 'hollow-bodied' animals are made up of two layers of cells more or less modified. But they are still of low organisation, one evidence of which is that, like the Protozoa, they have no vital parts, and that there is no separate canal for absorbing food and carrying away refuse, the mouth still opening direct into the body-cavity.

The lowest members of this sub-kingdom are the Sponges. They were long regarded as colonies of amœbæ, and therefore classed among Protozoa, but fuller knowledge of their structure as many-celled organisms, some of the highest among which show slight traces of nerves and sense-organs, has caused them to be ranked in a division called Porifera (Lat. porus, a pore; and fero, to bear). Very lovely are the skeletons which some of them secrete, such as Venus's flower-basket, with

its graceful fretted spirals; but more familiar to us are the useful fibrous and porous domestic sponges, woven of material said to be chemically allied to that spun by silkworms. Being rooted to one spot, the sponge-cells

A. Vertical section of outer layer magnified 75 times. p, pores or openings of canals for conducting water, which flows to a, sacs; e, canal for expulsion of water; g, early stages of spores.

B. Sac transversely divided (800 diameters), showing sponge-particles with cilia. C. Sponge-particle highly magnified. f, cilium; m, collar; n, nucleus; c, contractile vesicle.

have become specially modified for ingathering food and oxygen. Only the cells on the outside of the horny or flinty skeleton can procure food and oxygen easily; those living in the inside effect this by means of cilia,

the whip-like action of which drives the water, charged with food and oxygen, through the innumerable canals, whence, having served its purpose, it is driven out through other canals, carrying the refuse of the colony with it. The whole sponge represents, as has been aptly said, a kind of submarine Venice, 'where the people are ranged about the streets and roads in such a manner that each can easily appropriate his food from the water as it passes along.'

B

A

FIG. 39.-Hydra.

A. Planula or earliest stage of a hydroid on its emission from the

egg. B. Thread-cell

undisturbed. C. The

protruded.

Cilia also cover extensive surfaces of the higher animals. They abound about the eyes, the ears, the windpipe, and the brain of man; mysteriously moving independently of any other part, even of the nervous system, but fulfilling much less important functions than in the bodies of the lower animals.

Next in rank above the sponges are the tiny cup or tube-shaped, jelly-like, green-hued (because chlorophyll-containing) polyps named Hydra, colonies of which, with their bud-like clusters same with the filament of young-soon to start in life on their own account—are found clinging mouth downwards to weeds and rubbish in fresh water. From the mouth hang a number of tentacles containing cells, in which lie barbed threads coiled up in a poison-fluid. When anything touches these tentacles they contract, the cells burst and fling the thread, lasso-like, around the prey, poisoning it with the fluid. From some of the marine species which secrete tubes of flint, and project themselves therefrom like flowers, so that the sea depths are covered with their waving, plant-like forms, the buds detach themselves and become the beautifully tinted

Medusa or jelly-fish. These produce eggs which become rooted polyps, so that the offspring never resembles its parents, but always its grandparents. All living matter is largely made up of water, the average proportion ranging from seventy to ninety per cent., but in the jellyfish it is about four hundred to one. Yet, fragile as is the creature, its structure is complex. Canals traverse

the swimming-bell, and carry food and oxygen to every part; rudimentary muscles in the shape of contractile tissues propel the animal along in rhythmic grace of motion ; a nervous system runs round the margin of the bell; there are rudimentary eyes in bead-like pigmentspots, and rudimentary ears in small sacs along the margin; and the hanging tentacles are charged, as in its fresh-water ally, with deadly fluid.

Lovelier still, and of slightly more complex structure, are the variously coloured Sea-anemones, with their petal-like tentacles; while nearly allied to these are the

FIG. 41.-Vertical Section of Common Sea-anemone.

m, mouth; m', primary cavity; m", secondary cavity; e, ectoderm or outer skin; e', endoderm or inner skin; t, tentacle; t', ovary; d, disc of attachment; s, body-cavity.

The left side of the figure shows the coral denuded of soft parts; on the right the animal matter is shown, while at the upper part several of the polyps are seen projecting.

colonies of Coral-builders, which, despite the surging wave and drifting current, raise their tree-like structures, foundations of solid land on which the bird builds her nest and man sets his dwelling.