C.

Structure.

The thin, transparent, homogeneous external layer (cuticle).

B. The granular layer (cortical layer) inside the

8.

cuticle.

[Its fine transverse striation.]

The central more fluid part, not sharply marked off from B.

The various clear spaces (alimentary vacuoles) in it, containing foreign (swallowed) bodies (Diatoms, Protococcus, &c.).

The contractile vesicle; its position, in the cortical layer just beneath the disc; its systole and diastole.

E. The nucleus; an elongated curved body in the cortical layer; sometimes nearly homogeneous, sometimes more distinctly granular. The nucleus is usually indistinguishable until after treatment with iodine (4).

The gullet; sometimes seen in optical transverse section as a clear round space; sometimes seen sidewise as a canal opening above on the disc, and ending abruptly below in the body-substance.

b. The stalk.

α.

B.

Its length and diameter (measure).

Its structure; the external homogeneous layer (sheath) continuous with the cuticle; the highly refractive centre (axis) generally surrounded with granules, and continuous with the cortical layer of the bell.

3.

a. Its form; pear-shaped; rounded off above; no disc or peristome visible.

B. The clear transverse space near the top, indicating the interval between the retracted disc and the rolled-in peristome. In this space the cilia can frequently be seen moving.

b. The stalk; thrown into corkscrew-like folds.

The movements of Vorticella. Compare especially the regularity, definiteness and rapidity of some of them with the slow and irregular movements of Amaba. (III.)

a.

a.

γ.

The ciliary movement.

Examine the cilia carefully; delicate homogeneous processes; their length, diameter and form; their position.

[B. The continuity of the cilia with the cortical layer.] The function of the cilia; their rapid movements, alternately bending and straightening: the co-ordination of these movements; they work in a definite order; note the currents produced in the neighbouring water (if necessary introduce a few particles of carmine under the coverslip); the sweeping of small bodies down the gullet.

b. The movements of the contractile vesicle (see III. A. 3. c). Tolerably regular rhythmic distension and collapse (diastole and systole).

4.

5.

6.

C.

d.

a.

The currents in the central parts of the body carrying round the swallowed bodies. (Compare VI. C.)

The movements of the animal as a whole. (inch or inch obj.)

Its extreme irritability; it contracts on the slightest stimulation: often without any apparent cause.

B. The movements which occur in contraction; the coiling up of the stalk; the rolling in of the disc. The rapidity of these movements. The mode of re-expansion; the stalk straightens first; then the peristome is everted; finally the disc and its cilia are protruded.

Stain with iodine or magenta; the cuticle uncoloured -the rest stained; the nucleus especially becomes deeply coloured.

Treat with acetic acid; the contents soon disappear (except perhaps some swallowed bodies)-the cuticle later or not at all.

Note the following points in various specimens

a.

Multiplication by fission; a bell partially divided into two by a vertical fissure starting from the disc.

B. Two complete bells on one stalk; the result of completion of the fission. The development of a basal circlet of cilia by one or both of these bells.

ly. Free swimming unstalked bells (detached bells from B).]

B.

[8. Conjugation; the attachment of a small free swimming bell to the side of a stalked one.] [e. Encystation; the body contracted into a ball and surrounded by a thickened structureless layer, the contractile vesicle being persistently dilated.] Other forms closely allied to Vorticella which may be met with, and which will do nearly as well for examination, are ;

a.

b.

C.

Epistylis. Bell-shaped animals growing on a branched non-contractile stalk.

Carchesium. A form very like Vorticella but borne on a branched contractile stalk.

Cothurnia. An almost sessile form, provided with a cup or envelope into which the bell can be retracted.

[The activity of the movements of the free Infusoria interferes with the complete examination of the living animal. It is well therefore to add a little osmic acid solution to the drop of water under examination. This kills such Infusoria as Paramæcium, Nyctotherus and Balantidium instantly, without destroying the essential features of their organization.]

M.

X.

THE FRESHWATER POLYPES (Hydra viridis

and H. fusca).

to

From the

Ir a waterweed, such as duckweed, from a pond, is placed in a glass and allowed to remain undisturbed for a short time, minute gelatinous-looking bodies of a brownish or green colour may frequently be found attached to it, or to the sides of the glass. They have a length of from of an inch, and are cylindrical or. slightly conical in form. free end numerous delicate filaments, which are often much longer than the body, proceed and spread out with a more or less downward curve, in the water. If touched, these threads, which are the tentacles, rapidly shorten and together with the body shrink into a rounded mass. After a while, the contracted body and the tentacles elongate and resume their previous form. These are Polypes, the brown ones belonging to the species termed Hydra fusca, the green to that called H. viridis. The polypes usually remain attached to one spot for a long time, but they are capable of crawling about by a motion similar to that of the looping caterpillar; and, sometimes, they detach themselves and float passively in the water.

When any small animal, such as a water-flea, swimming through the water comes in contact with the tentacles, it is grasped, and conveyed by their contraction to the aperture