the case of the reproduction of Paramecium the beginnings. of developmental changes during the course of the growth of the young. The young Amoebae have only to add substance to their bodies, to grow larger, in order to be exactly like their parent.

The new Paramacia attain full size and then divide, each into two. And so on for many generations. But it has been discovered that this simplest kind of reproduction can not go on indefinitely. After a number of generations the Paramacia, instead of simply dividing in two, come

together in pairs, and a part of

one of the nuclei of each member of a pair passes into the body of and fuses with a part

FIG. 7.-Paramecium caudatum; two individuals separating after conjugation.

of one of the nuclei of the other member of the pair. In the meantime the second nucleus in each Paramecium has broken up into small pieces and disappeared. The new nucleus composed of parts of the nuclei from two animals. divides, giving each animal two nuclei just as it had before this extraordinary process, which is called conjugation, began (Fig. 7). Each Paramecium, with its nuclei composed of parts of the nuclei from two distinct individuals,

now simply divides in two, and a large number of generations by simple fission follow.

Paramecium in the character of its body and in the manner of the performance of its life processes is distinctly less simple than the Amaba, but its body is composed of a single structural unit, a single cell, and it is truly one of the "simplest animals."

6. Vorticella. Another interesting and readily found protozoan is Vorticella (Fig. 8). While the Amaba can crawl and Paramecium swim, Vorticella, except when very young,

A

B

FIG. 8.-Vorticella microstoma (after STEIN). A, small, free-swimming individuals conjugating with a large, stalked individual; B, a stalked individual dividing longitudinally; C, after division is completed one part severs itself from the other, forms a ring of cilia, and swims away.

is attached by tiny stems to dead leaves or sticks in the water, and can change its position only to a limited extent.

The body is pear-shaped or bell-shaped, with a mouth opening at the broad end, and a delicate stem at the narrow end. This stem is either hard and stiff, or is flexible and capable of being suddenly contracted in a close spiral. In the body mass there is one pulsating vacuole and one nucleus. Usually many Vorticelle are found together on a common stalk, thus forming a protozoan colony.

The life processes of Vorticella are of the simple kind already observed in Amœba and Paramæcium. Vorticella shows, however, some modifications of the process of reproduction which are interesting. The plane of division of Vorticella is parallel to the long axis of the pear-shaped body, so that when fission is complete there are two Vorticellæ on a single stalk. One of the two becomes detached, and by means of a circle of fine hairs or cilia which appear around its basal end leads a free swimming life for a short time. Finally it settles down and develops a stalk. Vorticella shows two kinds of fission-one the usual division into equal parts, and another division into unequal parts. In this latter kind, called reproduction or multiplication by budding, a small part of the parent body separates, develops a basal circle of cilia, and swims away. The process of conjugation also takes place among the Vorticella, but they are never two equal forms which conjugate, but always one of the ordinary stalked forms and one of the small free - swimming forms produced by budding.

Here, then, in the life of Vorticella, are new modifications of the life processes; but, after all, these life processes are very simply performed, and the body is like the body of the Amaba, a single cell. Vorticella is plainly one of "the simplest animals."

7. Gregarina.—A fourth kind of protozoan to which we can profitably give some special attention is Gregarina (Fig. 9), the various species of which live in the alimentary

canal* of crayfishes and centipeds and certain insects. Gregarina is a parasite, living at the expense of the host in whose body it lies. It has no need to swim about quickly,

FIG. 9.-Gregarinidæ. A, a Gregarinid (Actinocephalus oligacanthus) from the intestine of an insect (after STEIN); B and C, spore forming by a Gregarinid (Coccidium oviforme) from the liver of a guinea-pig (after LEUCKART); D, E, and F, successive stages in the conjugation and spore forming of Gregarina polymorpha (after KOELLIKER).

and hence has no swimming cilia like Paramecium and the young Vorticella. It does need to cling to the inner wall of the alimentary canal of its host, and the body of some species is provided with hooks for that purpose. The

* Specimens of Gregarina can be abundantly found in the alimentary canal of meal worms, the larvæ of the black beetle (Tenebrio molitor), common in granaries, mills, and brans. "Snip off with small scissors both ends of a larva, seize the protruding (white) intestine with forceps, draw it out, and tease a portion in normal salt solution (water will do) on a slide. Cover, find with the low power (minute, oblong, transparent bodies), and study with any higher objective to suit."MURBACH.

food of Gregarina is the liquid food of the host as it exists in the intestine, and which is simply absorbed anywhere through the surface of the body of the parasite. There is no mouth opening nor fixed point of ejection of waste. material, nor is there any contractile vacuole in the body.

In the method of multiplication or reproduction Gregarina shows an interesting difference from Amaba and Paramecium and Vorticella. When the Gregarina is ready to multiply, its body, which in most species is rather elongate and flattened, contracts into a ball-shaped mass and becomes encysted-that is, becomes inclosed in a tough, membranous coat. This may in turn be covered externally by a jelly-like substance. The nucleus and the protoplasm of the body inside of the coat now divide into many small parts called spores, each spore consisting of a bit of the cytoplasm inclosing a small part of the original nucleus. Later the tough outer wall of the cyst breaks and the spores fall out, each to grow and develop into a new Gregarina. In some species there are fine ducts or canals leading from the center of the cyst through the wall to the outside, and through these canals the spores issue. Sometimes two Gregarina come together before encystation and become inclosed in a common wall, the two thus forming a single cyst. This is a kind of conjugation. In some species each of the young or new Gregarinæ coming from the spores immediately divides by fission to form two individuals.

8. Marine Protozoa.—If called upon to name the characteristic animals of the ocean, we answer readily with the names of the better-known ocean fishes, like the herring and cod, which we know to live there in enormous numbers; the seals and sea lions, the whales and porpoises, those fish-like animals which are really more like land animals than like the true fishes; and the jelly-fishes and corals and star-fishes which abound along the ocean's edge. But in naming only these we should be omitting certain animals which in point