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 Lime 2. Starfish 3. Thistle down 4. Starch, Wheat 5. Do. Potato 6. Prawn-shell 7. Starch, “Tous les mois” 8. Bone, cancellous 9. Gun-cotton 10. Cow’s hair 11. Hoof, donkey, longitudinal 12. Do. transverse 13. Nitre, Crystals 14. Scale, Eel 15. Wing, Water-Boatman 16. Chlorate of Potash, Crystals 17. Cellularia reptans 18. Star-shaped hair, Stalk of Yellow Water-Lily 19. Teeth, Palate of Whelk 20. Zoophyte, Bowerbankia 21. Raphides, _i.e._ crystalline formations in vegetable cells, Bulb of Hyacinth 22. Do. Rhubarb 23. Sulphate of Magnesia, Crystals 24. Bone, Skate 25. Cherrystone, transverse section 26. Sugar, Crystals in honey 27. Tendon, Ox 28. Calcareous plates. Tooth of Echinus [Illustration: XI.] Many compound tree-like forms of Vorticella are known, one of which, _Carchesium_ (Fig. 15), may serve as a type of all. In the case of this organism, the colony contracts in sections on a moderate shock; in the second, _Zoothamnium_, as a whole; whilst in _Epistylis_ the stalks are rigid, and the individuals contract singly. When the shock is violent, the appearance presented by the two former is that shown in Fig. 16. In all three cases the colonies are usually so large that they are visible as trees to the naked eye, and some members of the group are extremely common. Moreover, they are often parasitic, as, for example, upon Cyclops, which is frequently loaded with them. Another compound form is _Ophrydium_, a colony of which (not unusually large) is shown of the natural size in Fig. 18, with a single zoöid, magnified, by the side of it, in Fig. 19.