1. The Protozoa, or One-celled Animals. 2. The Coelenterata or Two-layered Animals. 3. The Sponges or Porifera. 4. The Vermes or Worms. 5. The Arthropods or Jointed Animals, viz., Insects and Crustacea. 6. The Mollusca or Shell-fish. 7. The Brachiopoda or Lamp-Shells. 8. The Bryozoa or Moss-Corals. 9. The Echinodermata or Sea-Urchins. 10. The Chordata, including--(_a_) the Hemichordata; (_b_) the Ascidians; (_c_) the Vertebrata. Within recent years an attempt has been made to express the relationship these groups bear to one another, by placing them in separate divisions or grades. The first grade includes only the Protozoa, or unicellular animals. The position of second grade has been assigned to the Coelenterata or diploblastic animals, whose bodies consist typically of two layers of cells. A third grade includes only a few groups of the lower worms, among which three body-layers may be distinguished, but no body-cavity is present. While the fourth grade, including practically the rest of the animal kingdom, have three body-layers (see p. 38), and a body-cavity surrounding the internal organs (see p. 38). This arrangement of groups is an extremely convenient one; all the more convenient because it easily admits of modification. Already, indeed, we might find room for a grade intermediate between I. and II., consisting of what might be termed monoblastic animals, namely, animals consisting of a single layer of cells. For the frequent occurrence of Larvæ of this kind, consisting of a hollow ball of cells, renders zoologists on the alert to find a grown-up organism built in the same way. It is doubtful whether any of the forms that have been supposed to answer to this description really do so. Certain forms of these often claimed as plants by the botanists are, however, in the meanwhile, invited in to fill the blank. There are also animals in which the internal layer of the body is very much reduced, consisting sometimes in fact of one cell only. Those are the Dicyemidæ and Orthonectidæ, both of them parasitic forms. They differ so completely from all other forms that it has been proposed to make for them a special group, the Mesozoa, or Midway animals, between the Protozoa and all the rest of the animal kingdom. It is, however, possible to group them under the head of Diploblastic animals; but nothing more different from the Coelenterata could well be imagined, and some regard them as a degraded form of worm. The animals which are higher in structure than the Protozoa, viz. our divisions 2 to 10, are often grouped under the name Metazoa. The Metazoa thus include Grades II., III., and IV. The meaning of the division of the animal kingdom into grades will be more apparent if we give an example of each.