DEFINITION.—We may now define sponges as multicellular, {181}two-layered animals; with pores perforating the body-walls and admitting a current of water, which is set up by the collared cells of the "gastral" layer. POSITION IN THE ANIMAL KINGDOM.—Sponges are the only multicellular animals which possess choanocytes, and their mode of feeding is unique. Since they are two-layered it has been sought to associate them with the Metazoan phylum Coelenterata, but they are destitute of nematocysts or any other form of stinging cell, and their generative cells arise from a class of embryonic cells set apart from the first, while the generative cells of Coelenterata are derived from the ectoderm, or in other cases from the endoderm. These weighty differences between sponges and that group of Metazoa to which they would, if of Metazoan nature at all, be most likely to show resemblance, suggest that we should seek a separate origin for sponges and Metazoa. We naturally turn to the Choanoflagellate Infusorian stock (see p. 121) as the source of Porifera, leaving the Ciliate stock as the progenitors of Metazoa. That both Porifera and Metazoa are reproduced by ova and spermatozoa is no objection to this view, seeing that the occurrence of similar reproductive cells has been demonstrated in certain Protozoa (see pp. 100, 128). Let us now see which view is borne out by facts of embryology. Suppose, for the moment, we regard sponges as Metazoa, then if the sponge larva be compared with the Metazoan larva we must assign the large granular cells to the endoderm; the flagellated cells to the ectoderm; and we are led to the anomalous statement that the digestive cells in the adult are ectodermal, the covering, outer cells endodermal; or conversely, if we start our comparisons with the adults, then it follows that the larval ectoderm has the characters of an endoderm, and the larval endoderm those of an ectoderm. Thus both embryology and morphology lead us to the same point, they both show that in the absence of any fundamental agreement between Porifera and Metazoa it is necessary to regard the two stocks as independent from the very first, and hence the name PARAZOA (Sollas) has been given to the group which contains the Porifera as its only known phylum. Interesting in connexion with the phylogeny of Parazoa is the Choanoflagellate genus _Proterospongia_ (Fig. 75), described by {182}Saville Kent, and since rediscovered both in England and abroad.[211] This is a colony of unicellular individuals embedded in a common jelly. The individuals at the surface are choanoflagellate, while in the interior the cells are rounded or amoeboid, and some of them undergo multiple fission to form reproductive cells. This is just such a creature as we might imagine that ancestral stage to have been of which the free-swimming sponge larva is a reminiscence: for we have seen that the flagellated cells of the larva are potential choanocytes.