The term Zoophytes, or animal-plants, comprises two very distinct classes of living beings, namely, the Porifera, or Sponges, which (if not vegetables) are wanting in many attributes regarded as essential characteristics of the members of the animal kingdom; and the Polypifera, or polype-bearing-animals,--the Corals; which are generally associated groups or aggregations of individuals, united by a common organized mass or axis, each polype having an independent existence, and exhibiting volition and perception, in a greater or lesser degree. Fossil Porifera. [Sidenote: FOSSIL PORIFERA.] The terms _Amorphozoa_ (signifying animals of variable shapes), and _Porifera_ (structures traversed by pores or channels), are employed by naturalists to designate the Sponges and analogous organisms, which appear to occupy the boundary line that separates the animal from the vegetable kingdom. The true position of the Sponges in the great system of Creation is still a disputed point; for while many distinguished naturalists regard them as _Protozoa_, or the lowest type of animal organization, others of equal eminence affirm that neither in structure nor functions do they differ from vegetables in any essential particular; and that if a line be drawn between the two kingdoms the Porifera must be placed on the vegetable side of the boundary. On the other hand, Dr. George Johnston, in his delightful work on the British Zoophytes,[193] expresses his opinion that there is nothing to discountenance the belief that these bodies hold an intermediate place; that they are, in fact, the true _Zoophytes_, or animal-plants; in some forms, as the green _Spongillæ_ of our lakes, the vegetable nature prevails; while in others, as the horny or keratose sponges filled with mucilaginous slime, and the fleshy _Tethya_ whose oscula, or pores, are said to exhibit signs of irritability, the animal character predominates. [193] A History of British Sponges, &c. by Dr. George Johnston, Edinburgh. 1843. One vol. 8vo. with twenty-five plates. A previous work, "A History of British Zoophytes," with forty-four plates, from drawings by the accomplished lady of the Author, cannot be too highly commended. Without committing ourselves to either opinion, and simply remarking that the large proportion of silex that enters into the tissues of a considerable number of the porifera, is more characteristic of vegetable than of animal structures, we proceed to consider the fossil sponges and allied forms, as the mineralized remains of the lowest types of the animal kingdom: if the vegetable nature of the originals were generally admitted, this section should have followed that which treats of the Diatomaceæ (_ante_, p. 100).