2. _Rotifer vulgaris_: _a_, jaws and teeth; _b_, antenna; 2*, wheel-organ expanded. 3. _Pterodina patina._ 4. _Floscularia ornata._ 5. _Hydra viridis._ 6. _Arcella vulgaris._ 7. _Arcella aculeata._ 8. _Arcella aculeata_, shell with animal. 9. _Arcella dentata._ 10. _Amœba diffluens._ 12. _Actinophrys sol._ 13. Sponge, fibres of; 13 _a_, _b_, _c_, spicules of Sponge. 14. _Sertularia pumila_, polypidom. 15. _Sertularia pumila_, polypidom with polypes. 16. _Monas lens._ 17. _Cercomonas globulus._ 18. _Cercomonas crassicauda._ 19. _Heteromita ovata._ 20. _Anthophysa Mülleri._ 21. _Dinobryon sertularia._ 22. _Trachelomonas volvocina._ 23. _Chætoglena volvocina._ 24. _Euglena viridis._ 25. _Astasia hæmatodes._ 26. _Enchelys nodulosa_; _a_, undergoing transverse division. 27. _Oxytricha gibba_; 27 _a_, side view. 28. _Paramecium aurelia_: _a_, contractile vesicle; _b_, a gastric sacculus. 29. _Amphileptus fasciola._ 30. _Colpoda cucullus_: _a_, contractile vesicle. 31. _Nassula elegans_: _a_, vesicle; 31 _b_, encysted form. 32. _Coleps hirtus._ 33. _Vaginicola crystallina._ 34. _Vorticella convallaria_: _a_, stalk spirally contracted; _b_, body undergoing longitudinal division. 35. _Vorticella convallaria_ encysted and discharging the young brood. 36. _Vorticella convallaria_, body with nucleus (_a_). 37. _Chilodon cucullulus._ 38. _Stentor polymorphus_: _a_, body extended; _b_, body contracted; _c_, bodies aggregated around a globule of jelly; _d_, bodies adherent to the side of a glass. 39. _Alyscum saltans._ 40. _Podophrya fixa_, or the _Podophrya_-form of _Vorticella_. [Illustration: Plate XI. W Bagg sculp London: John Van Voorst.] _Pterodína pat´ina_ (Pl. XI. fig. 3).--This species has a shell or carapace on the back, a two-lobed rotatory organ, two eyes, and a slender wrinkled tail ciliated at the extremity. The curved alimentary canal, and the two strong muscles inclined at an angle, are easily distinguishable. _Flosculária ornáta_ (Pl. XI. fig. 4) is a very beautiful member of the Rotatoria, and is found adhering to _Confervæ_ and other water-plants. The body is club-shaped, and contained in a transparent tube, the ringed narrower portion being fixed to its base. The rotatory organ is divided into five or six lobes, furnished with long, slender, radiating tentacular filaments; these are not vibratile like ordinary cilia, but can be slowly moved. In the contracted state, the filaments form a pencil-like bundle.