66

paratively smooth. Tubipora, or organ-pipe coral," consists of smooth red tubes connected at intervals by cross plates. The Astræa, Meandrina, Madrepora, and Porites are the chief reef-forming corals. They will not live in waters whose mean temperature in the coldest month is below 68° Fahr., nor at greater depth than about twenty fathoms. The most luxuriant reefs

are in the central and western Pacific and around the West Indies.

A coral reef is formed by many corals growing together. It is to the single coral stock as a forest is to a tree. The main kinds of reefs are fringing, where the reef is close to the shore; barrier, where there is a channel between reef and shore; encircling, where there is a small island inside of a large reef; and coral islands,

or atolls, where there is simply a reef with no land inside of it. The Great Barrier Reef off the east coast of

Australia is 1250 miles in length. All reefs begin as

fringing reefs, and are gradually changed into the other

forms by the slow sinking of the bottom of the ocean, or by the death, decay, and disintegration of the corals on the landward side of the reef, where the food supply is necessarily restricted.

FIG. 35.

Sea Fan (Gorgonia) and Sea Pen (Pennatula).

(2) Sclerobasic Corals.- Corallium rubrum, the precious coral of commerce, is shrublike, about a foot high, solid throughout, taking a high polish, finely grooved on the surface, and of a crimson or rose-red color. In the living state the branches are covered with a red cœnosarc studded with white polyps (Fig. 34).

which work like paddles. The body is not contractile, as in the jellyfishes. They are considered the highest of colenterates, having a complex nutritive apparatus and a definite nervous system. There is no trace of a polyp stage in their development, and they do not form colonies. They are found in all regions of the ocean, from the arctics to the tropics.

Branch IV. - PLATYHELMINTHES

The group formerly called Vermes or worms was composed of animals so very different in form and structure that it has now been subdivided into several branches, viz.: Platyhelminthes, or flat worms; Nemathelminthes, or round worms; Trochelminthes, or rotifers; Molluscoida, including the Polyzoa and Brachiopoda; and Annulata, or segmented worms. All these forms agree in being distinctly bilaterally symmetrical animals, as contrasted with the apparently radial arrangement of parts seen in the Calenterata and Echinodermata, and in having the three body layers -ectoderm, mesoderm, and endoderm — well developed, the mesoderm or middle layer being relatively of more importance than in any preceding group.

The Platyhelminthes, or flat worms, include some free forms, as Planaria, which is common in fresh water, and the tapeworms and flukes among the parasites. As a group, they are soft-bodied, flattened animals, without skeletal parts of any kind. There is no distinct body cavity nor blood-vascular system nor anal opening. The digestive system may be entirely absent, as in the tapeworm, or it may be much branched and highly complicated in structure, as in the planarians.

The tapeworm (Tania) consists of the so-called head and the body segments, which are really reproductive

joints. It develops from the egg in the digestive canal of the pig, burrows into the muscular tissue of the animal, and there becomes encased.

b

a

Pork containing these cysts is called "measly pork." If the pork be eaten by man, in an uncooked condition, this case is dissolved by the gas

d

tric juice, and the embryo thus released attaches itself to the intes

FIG. 37. — Tapeworm (Tania solium): a, head; b, c, d, segments of the body.

FIG. 38. Planarian
Worm.

tine by its "head," and develops into the tapeworm by budding off the reproductive segments, or proglottides. As these become ripe and filled with fertilized eggs, they are detached, and pass off with the excrement.

The disease called "rot," in sheep, is produced by the fluke (Distoma), which grows in the bile ducts of the sheep.

The flat worms are the most widely distributed of all animals above the Protozoa. They are found on land, at various depths in bodies of fresh water, and in the They also occur as parasites in animals in almost every class of the Metazoa.

sea.