surface of the water. this is not the case.

It can be shown, however, that

If a large beaker filled with

water be placed inverted in the aquarium, the Cucu

19.

beaker begin to creep up to

mariæ that are under the the bottom of the glass. They also do so when the experiment is made in the manner represented in Fig. A bridge BB is placed in the aquarium A A, the horizontal part of the bridge B B being below the surface of the water of the aquarium. The hori zontal part has a round opening o over which the inverted beaker a bed filled with water is placed. Fresh water is supplied at a low pressure at o through a glass tube g, which has been properly bent. The Cucumariæ nevertheless go away from o and remain at the highest point cd, or near cd on the vertical sides (Fig. 19), where they ultimately die.

Experiments on the centrifugal machine yielded no result, for the animals did not move during the rotation. Gravity is the only condition which can account for the phenomenon, and I imagine the influence which gravity exercises to be in a manner similar to that observed among insects-for example, in butterflies which have just emerged from the chrysalis. The wings of the butterfly do not unfold immediately, and it runs about restlessly until it comes to a vertical surface. When this is reached, the butterfly creeps upon it and remains there for some time with its head up. After the wings are spread, other conditions cause the animal to be restless again.

Because of this dependence on gravity, the Cu

cumariæ are of necessity inhabitants of the surfaceregions of the ocean. If a larva were carried down

to a great depth, its negative geotropism would force it to migrate upward until the highest point was reached or until death put an end to its upward journey.

Certain starfish - for instance Asterina gibbosa, which also lives near the surface of the water-behave like Cucumaria. All the experiments I have made on Cucumaria can likewise be successfully performed on Asterina gibbosa, but with the difference that the exceptionally voracious Asterina does not remain permanently at the highest point of the vertical surface. In two days, or sometimes even sooner, it begins to move or drops down.

Positive heliotropism naturally has the same effect as negative geotropism. Asterina tenuispina, like Asterina gibbosa, lives at the surface of the sea. It is not, however, geotropically irritable; but it is positively heliotropic. I put a large number of specimens of both species in a heap in an aquarium, into which rays of light from one side only fell nearly horizontally. In a short time the two species had parted, the Tenuispinæ crawling off on the floor toward the source of light. The Gibbosa, scattered about on the bottom of the aquarium in every direction, crawled up the vertical sides without being influenced at all by the light in their movements. In the ocean, where the vertical rays of daylight are chiefly concerned in the orientation of animals, positive

heliotropism must drive Asterina tenuispina to the surface of the ocean, just as Asterina gibbosa is driven there by negative geotropism.

Preyer mentions briefly in his extensive work on The Movements of the Starfish the "tendency of these animals to move upwards." "The strong tendency of starfish and brittlestars to go upward cannot be traced back to lack of air, lack of food, changes in temperature or current, or to a desire for light, for they climb up just the same when these conditions are eliminated. Probably some peculiarity of the bottom, or of just that part of the bottom where the animal is, makes it unsuitable for the suction of the tube-feet. The animals can remain there no longer, so they move upwards. But it is possible that parasites, which I have often found in the ambulacral furrows, may cause this upward migration, for as the stimuli produced by them come from below, they might seem to belong to the bottom."

The first sentence in this generalisation is wrong; the light attracts Asterina tenuispina upwards. Second, the character of the bottom does not determine the phenomenon. If Asterina gibbosa be placed in a cubical box with glass sides, the animals leave the basal horizontal side and crawl up the vertical sides. If the box then be turned 90° around a horizontal axis, the side which is now basal is deserted by the animals. They crawl up and remain on the side which, while horizontal, they had left. Finally, if Preyer believed that parasites force the animals to

crawl upward, it is difficult to see why they should not drive the animals down from the vertical side. As a fact, however, Asterina gibbosa, as well as Cucumaria cucumis, remains on the highest point of the vertical side. I believe it is much nearer the truth to ascribe the vertical upward movements of certain starfish to an action of the force of gravity.

BIBLIOGRAPHY.

1. ROMANES, G. J. Felly-fish, Starfish and Sea Urchins. New York, 1893.

2. PREYER, W. Ueber die Bewegung der Seesterne. Mittheilungen aus der zoologischen Station zu Neapel, Bd. vii, S. 96.

3. LOEB, J. Ueber Geotropismus bei Thieren. Pflüger's Archiv, Bd. xlix, 1891.

CHAPTER VI

EXPERIMENTS ON WORMS

1. We shall consider separately in this chapter two kinds of worms: first, those in which the ganglia are all crowded together in

chii), Fig. 20, a marine

FIG. 20. THYSANOZOON BROCCHII, A Planarian, is very com

MARINE PLANARIAN.

g, brain; m, mouth; n, longitudinal nerve. (Diagrammatic after Lang.)

mon in the Bay of Naples. It is from 1 to 3

cm. long and nearly as broad. The oral end of the body, which can be recognised by two tentacles