a. Large somewhat conical nucleated cells, with the broader end turned outwards.

B. Smaller rounded cells packed between the deep ends of the larger ones.

7. The nematocysts: small oval capsules, with a filament coiled up inside them, which are dispersed

through the ectoderm in the interior of its component cells.

4. Treat with magenta: note the staining of the cells, the emission of the thread-cells, and the protrusion of their threads: three chief forms of thread-cell

а. An oval capsule with a filament many times its own length attached to one end, and three short processes radiating from the base of the thread. B. Smaller thread-cells, without the radiating processes and with a short thread.

γ. Cells like ẞ, but with a much longer thread.

5. Imbed in paraffin a Hydra which has been hardened in chromic or osmic acid1 and cut sections from it; or lay a prepared Hydra on a glass slide and with a razor cut off transverse slices; having obtained by either method a number of thin sections mount them in glycerine and make out

The large and small cells of the ectoderm and its thread-cells, their arrangement and relations. (3).

1 When a Hydra is placed in the above hardening fluids it nearly always contracts so much as to make it difficult to cut sections. If it be first killed, by placing it in a small quantity of water and when it has expanded adding some boiling water, fairly extended specimens for hardening can usually be obtained.

B. The cells of the endoderm: large, nucleated, with a flattened base and a rounded free end: their arrangement in a single layer.

The thin intermediate layer (muscular stratum) between ectoderm and endoderm.

8. The body cavity.

6. Tease out in water a specimen which has been treated with weak chromic acid (o. 10) or with osmic acid: make out the various cells already described : notice branched tails proceeding from the narrower ends of the larger ectoderm cells.

17. Tease out a fresh Hydra in water and observe the various cells. Note the amoeboid movements exhibited by some, and the single cilium attached to other (endoderm) cells.]

8. Gently flatten out a testis in water by pressure on the coverslip, and examine with a high power. According to its state of maturity the following contents will be found in it—

a.

A collection of the smaller ectoderm cells.

B. The same but having lost their nucleus and become hyaline.

7. Cells otherwise like ß, but with a long filament proceeding from them.

8. Ripe spermatozoa: bodies consisting of a very small oval head to which a very delicate filament is attached, and which, should they get free, swim about in the water by the movements of this filament. They may frequently be seen in motion within the unruptured testis.

9.

Press out an ovary: according to its stage of development there will be found in it—

а.

γ.

Simply ectoderm cells with an unusual preponderance of the smaller form.

B. Imbedded among cells like a, one which has become larger and clearer than the rest, and possesses a distinct central clear spot in it. Considerable aggregation of granular protoplasm round this cell, so as to form a body consisting of a granular protoplasmic mass, in which is imbedded a clear round vesicle, which again contains a distinct rounded dot.

8.

E.

The ripe ovum. Consisting of a great irregularly branched mass of protoplasm (vitellus), in which is a clear space (germinal vesicle) containing another body (the germinal spot). The segmented ovum: composed of a large number of small cells. Its thick capsule, rough on its external surface.

XI.

THE FRESH-WATER MUSSEL

(Anodonta Cygnæa).

UNDER the name of 'Fresh-water Mussel' two distinct kinds of animals, which are not unfrequently abundant in our ponds and rivers, are included; namely, the Anodonta and two or three kinds of Unio. The Anodonta is chosen for special study here, but what is said about it applies very well to all parts of Unio except the shell.

The animal is enclosed in a shell composed of two pieces or valves, which are lateral, or right and left, in relation to the median plane of the body. The more rounded and broader end is anterior, the more tapering, posterior. If placed in a vessel of water, at the bottom of which there is a tolerably thick layer of soft mud or sand, and left quite undisturbed, the Anodonta will partially bury itself with its anterior end directed obliquely downwards; and the valves will separate at their ventral edges for a short distance. At the edges of this 'gape' of the shell the thickened margins of a part of the contained body which is called the mantle, become visible, and between them a large, whitish, fleshy, tongue-shaped structure-the foot-not unfrequently protrudes, and is used to perform the sluggish movements of which the Anodonta is capable. If some finely dividing colouring matter, such as indigo, is dropped into the water,

so as to fall towards the gape, it will be seen to be sucked in; while, after a short time, a current of the same substance will flow out from an opening between the two edges of the mantle on the dorsal side of the posterior end of the body; and these 'inhalent' and 'exhalent' currents go on, so long as the animal is alive and the valves are open. Any disturbance, however, causes the foot, if it was previously protruded, to be retracted, while the edges of the mantle are drawn in and the two valves shut with great force. On the other hand, in a dead Anodonta the valves always gape, and if they are forcibly shut spring open again. The reason of this is the presence of an elastic band, which unites the dorsal margins of the two valves, for some distance, and is put on the stretch when the valves are forcibly brought together. During life they are thus adducted by the contraction of two thick bundles of muscular fibres, which pass from the inner face of one valve to that of the other, one at the anterior and the other at the posterior end of the body, and are called the anterior and posterior adductors.

The animal can be extracted from the shell without damage, only by cutting through these muscles close to their attachments. It is bilaterally symmetrical, the foot proceeding from the middle of its ventral surface; the mouth is median and situated between a projection, which answers to the under surface of the anterior adductor muscle, and the superior attachment of the foot. On each side of the mouth are two triangular flaps with free pointed ends-the labial palpi-and behind these, on each side, two broad, plate-like organs, with vertically striated outer surfaces, are visible. These are the gills or branchiæ. In the dorsal region, the integument is soft and smooth; on each side, it is produced into large folds, the lobes of the mantle or pallium, which closely adhere to the inner surface of the valves of the shell,