The blood of animals is analogous in its office to the sap of plants, but differs greatly from it under the microscope. In sap there seem to be no microscopic characters, except that when a branch is cut, as in the vine, the flowing sap may contain certain substances formed in the wounded cells, such as chlorophyll, starch, and raphides; but the blood is known to be an exceedingly complex substance both in a microscopic and a chemical point of view. When a little fresh blood is placed under the microscope, it is seen to consist of a colourless fluid filled with numerous little bodies, commonly called “blood-globules,” varying very greatly in size and shape, according to the animal from which they were taken. Those of the reptiles are very large, as may be seen at Fig. 4, Plate X., which represents a blood corpuscle of the Proteus. In this curious reptile the globules are so large that they may be distinguished during its life by means of a common pocket lens. In the vertebrated animals these corpuscles are red, and give to the blood its peculiar tint. They are accompanied by certain colourless corpuscles, spherical in form, which are sometimes, as in man, larger than the red globules, and in others, as in the siren and the newt, considerably smaller. The general view of the red corpuscles has sufficient character to enable the practised observer to name the class of animal from which it was taken, and in some cases they are so distinctive that even the genus can be ascertained with tolerable certainty. In point of size, the reptiles have the largest and the mammalia the smallest, those of the Proteus and the musk-deer being perhaps the most decidedly opposed to each other in this respect. IX. FIG. 1. Amœba diffluens 2. Arcella 3. Sun animalcule 4. Miliolina 5. Paramœcium 6. Chilodon subdividing 7. Melicerta ringens 8. Spicula of Sponge, Grantia 9. Noctiluca miliaris 10. Rotifer vulgaris 11. Do. jaws 12. Sponge animalcule 13. Sertularia operculata 14. Sponge, Grantia 15. Sertularia operculata, with ovicells 16. Actinia, showing weapons 17. Do. base of weapon more magnified 18. Sponge granule, ciliated 19. Anguinaria anguina 20. Spicules of sponge from Oyster Shell 21. Head of Snake-headed Zoophyte 22. Bugula avicularia 23. Polyzoön, Eretea 24. Do. Notamia 25. Zoea, Young of Crab 26. Hydra tuba 27. Medusa, cast off from above 28. Naked-eyed Medusa, Thaumantias 29. Compound Eye, Medusa 30. Larva, Snake Star 31. Water Flea 32. Serpula, Pushing Pole 33. Comatula, early stage of Starfish 34. Carbonate of Lime, artificial 35. Sea Urchin, transverse section of spine 36. Serpula, bundle of spears 37. Sun-star, part of skin 38. Oyster shell in different stages 39. Cilia on mussel [Illustration: IX.]