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of freeing themselves from their stalk and swimming

away to another place where the new colony is to be

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started.

Thus there is

shown among these simple organisms the differentiation of parts which is one of the characteristic features of the higher forms of animal life.

The cilia are used for d locomotion and for obtaining food, which, except in the parasitic species, is in the form of solid particles, consisting of bacteria and micro

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FIG. 11. — Vorticella: a, showing stages of scopic plants and animals, or of minute fragments of

fission, and b, internal structure; d, ciliated disk; g, gullet; n, nucleus; c, con

magnified.

tractile vacuole; f, food vacuole. Much animal and vegetable material. All of the ciliata which ingest solid food have a permanent mouth opening. Tentaculifera

suck through their tentacles the soft material composing the bodies of their prey. The cilia are uniformly arranged over the body, as in Paramecium, or are restricted to definite regions, as in Vorti cella. In either case

FIG. 12.- Acineta, animal in its lorica, 7, showing suctorial tentacles and nucleus, n, with contractile vacuole. Only a small part of the stalk is shown. Magnified.

there may be variations in their form, size, and function. Reproduction is by division and by budding,

spore formation being exceptional. It has been estimated that by self-division a Paramecium may give rise to 1,364,000 in forty-two days (Figs. 10, 11).

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The Metazoa include all those animals whose bodies are multicellular, which reproduce by true eggs and spermatozoa. This series includes eleven of the branches of the animal kingdom.

Branch II. PORIFERA

The position of the sponges has been much disputed. At first they were thought to be on the border line between animals and plants, and were assigned by some to the animals and by others to the vegetables. Later, and up to very recent years, they were assigned to the Protozoa. The discovery

of their mode of reproduction and development has determined that they belong to the Metazoa.

Simple sponges, like Grantia (Fig. 13), are somewhat vaseshaped in outline, and have a single central cavity communicating with the outside through an opening called the osculum. The wall of the body is pierced by numerous fine canals which communicate more or less directly with the central cavity on the one hand and with the exterior on the other. There is no body cavity. The body wall is composed of the skeleton, together with the cellular elements forming the "flesh." The sur

DODGE'S GEN. ZOÖL.-5

FIG. 13. - Diagram of a simple sponge: i, inhalant opening; o, exhalant opening or osculum.

face of the body is covered by a single layer of flattened cells forming the ectoderm. The canals are more or less completely lined with a layer of cells, each of which is provided with a flagellum, by means of which water is propelled through the canals toward the central cavity. Between these two layers is a mass of amoeboid and other cells which compose the mesoderm, and in which the skeleton, or framework, of the sponge is developed. The skeleton may be composed of flexible

FIG. 14.

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n

"Collar showing flagellum,

f; "collar," c; nucleus, n; contractile vacuole, cv. Much magnified.

fibers of spongin, as in the toilet sponge; or of spongin fibers together with spicules of calcareous matter, as in Grantia; or of siliceous spicules alone, as in the fresh-water sponge (Spongilla and Myenia) and Venus's flower basket (Euplectella); or of spicules of carbonate of lime and so on, while a few have no skeleton at all.

The flagellate cells are peculiar, in that they have an upgrowth on their free end, formed by a delicate expansion of the cell substance, and having the shape of a broad collar surrounding the base of the flagellum, whence their name of "collar cell" (Fig. 14).

The water flowing in through the canals bears with it the small particles of organic material upon which the sponge feeds, the particles being captured apparently by the collar cells as well as by the amoeboid cells. The same currents of water serve also for respiration. Reproduction is by means of eggs and by budding, the latter process giving rise to a group of connected sponges. The young larval sponge which develops from the fertilized egg is provided with cilia, by means of which it can swim around

for a time. Later, it comes to rest, attaches itself to some support, and develops into the adult form which is

a

a

FIG. 15. Hypothetical Section of a Sponge : a, superficial layer; b, inhalant pores; c, ciliated chambers; d, exhalant aperture, or osculum; e, deeper substance of the Sponge.

never capable of locomotion. The fresh-water sponges also multiply by means of gemmules, which are small,

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seedlike bodies to be found in the sponge in the fall. Each consists of a hard coating surrounding a mass of

cells and food substance. These gemmules survive the winter, and in the spring the cellular contents come out and develop into a sponge.

The sponge individual contains one exhalant orifice (osculum), with the channels leading into it. An ordinary bathing sponge constitutes a colony of such individuals, which are not definitely marked off from each other. Some other sponges have only one osculum, and such are a single individual, e.g. Grantia.

Excepting a few small fresh-water species (as Spongilla), sponges are marine. In the former, the cellular part is greenish, containing chlorophyll; in the latter, it is brown, red, or purple. In preparing the sponge of commerce, this is rotted by exposure, and washed out. The best fishing grounds are the eastern end of the Mediterranean and around the Bahama Islands.

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In the animals comprising this group, the body cavity is not distinctly separated from the digestive cavity. The cœlenterates are almost wholly marine forms, — hydroids, corals, sea anemones and jellyfishes,but there are a few which, like Hydra, live in fresh water. The body is usually radially symmetrical and shows three more or less definite cell layers, the ectoderm on the outer surface, the endoderm lining the inner cavities, with the mesoderm, or middle layer, between the others. In hydra and the hydroids the mesoderm is reduced to a mere film, but in the jellyfishes and sea anemones it forms a large part of the body. A characteristic feature is the presence of the stinging cells, or nematocysts, which are almost invariably to be found except in one group, Ctenophora, - where they

are replaced by adhesive cells.

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