The Trachelius (Fig. [8]) is perhaps the largest of all the Infusoria, being readily visible to even an inexperienced eye. Its body is richly furnished with contractile vesicles, and the protoplasm is curiously reticulated. We may here remark that the Trachelius is especially prompt in doing what most of these organisms do when put under pressure in a live-box, namely, in performing a kind of harakiri. The outline first becomes irregular, then the body rapidly swells and finally comes to pieces, the fragments dancing mockingly away under the influence of their still-moving cilia. The remedy is to use the cotton-wool trap and the lightest possible pressure.

A very elegant organism is shown in the bottom right-hand corner of the Plate (Fig. [25]). It is the Loxophyllum, and has a strongly marked contractile vesicle.

Another large form is Amphileptus (Fig. [9]), already referred to as having a large number of contractile vesicles arranged in a regular row; and more massive still is Bursaria (Fig. [10]), a very curious organism, very much like a purse indeed, and possessing a wonderful arrangement of cilia inside the funnel. These are arranged like a ladder, a series of rows of short stiff cilia, which move at short intervals in unison, and tend to sweep down into the cavity any small particles of food. This arrangement is here described for the first time, and appears to be quite unlike anything else among the Infusoria. Not unlike Bursaria, but having no ladder, and being furnished with a delicate membranous pouch in front of the slit of the purse, is Condylostoma, which we shrewdly suspect to be the young form of Bursaria. This is a point which requires elucidation.

One of the most beautiful of all these forms is shown in Fig. [11], Folliculina, a type of a large group characterised by the possession of a transparent case, of extremely elegant form, within which the animal retreats on the slightest alarm.

Fearless and independent, as becomes its size, is the trumpet-shaped Stentor (Fig. [12]), which may easily be seen when present, as it is in almost every good gathering of water-weed. The particular form drawn (S. Mülleri) does not make a case, but many members of the genus do, and it is very common to see a stem almost covered with them. Such a sight, once seen under dark-field illumination, will never be forgotten. The method of multiplication of the Stentors (by division) is extremely easy to watch, and very instructive.

A curious organism is Trichodina (Fig. [13]), which, though a free-swimmer, is always parasitic upon the body of some higher animal. We have found it sometimes upon Hydra, and always in hundreds upon the stickleback. The next group of Infusoria is distinguished by the body’s being only ciliated at particular points, usually round the mouth, or what acts as such. The first form is Vorticella (Fig. [14]), a beautiful vase-like creature upon a stem. Down the stem runs a muscular fibre, and on the least shock the fibre contracts and draws the stem into a beautiful spiral, whilst the cilia are drawn in, and the zoöid assumes the appearance of a ball at the end of a watch-spring. An exquisite sight is a colony of Vorticellæ, for these actions are always going on, as, for example, when one member of the family touches another, which is quite sufficient to provoke the contraction.

XI.

POLARIZED LIGHT.

FIG.
1.Carbonate of Lime16.Chlorate of Potash, Crystals
2.Starfish17.Cellularia reptans
3.Thistle down18.Star-shaped hair, Stalk of Yellow Water-Lily
4.Starch, Wheat19.Teeth, Palate of Whelk
5. Do.  Potato20.Zoophyte, Bowerbankia
6.Prawn-shell21.Raphides, i.e. crystalline formations in
7.Starch, “Tous les mois”   vegetable cells, Bulb of Hyacinth
8.Bone, cancellous22. Do.  Rhubarb
9.Gun-cotton23.Sulphate of Magnesia, Crystals
10.Cow’s hair24.Bone, Skate
11.Hoof, donkey, longitudinal 25.Cherrystone, transverse section
12. Do.  transverse26.Sugar, Crystals in honey
13.Nitre, Crystals27.Tendon, Ox
14.Scale, Eel28.Calcareous plates. Tooth of Echinus
15.Wing, Water-Boatman