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Show 1886.] POSITION AND CLASSIFICATION OF SPONGES. 567 boid cells met with in the Sponges are invariably modified cells mesoglcea. This is particularly striking and important in the case of the muscular and sensitive elements. The Epithelaria, on the other hand, have a mesogloea the cells of which remain more or less amoeboid and are not differentiated to any extent. The muscular, glandular, sexual, sensitive, ganglionic and defensive nettle-cells are produced in the epithelia, they sink below the outer cell-layer with advancing development and lie on the surface of the mesogloea or supporting lamella. By a process of folding and subsequent coalescing of the fold-margins, bundles of muscular cells may become immersed in the mesoglcea, and so form a mesodermal structure, which, however, must be considered a secondary mesoderm, as compared to the primary mesoderm represented by the mesoglcea and its cells. But they are invariably produced first from the epithelia and immersed afterwards, and always retain their epithelial character in clothing the walls of tubular cavities in the mesogloea. Single muscular cells are never surrounded on all sides by the mesoglcea. Solid bundles of muscular cells do not occur. Exceptionally nettle-cells (Cram-bessa) may be found in the mesoglcea, which is also here and there traversed by nerve-fibres (Cycloneurous Medusa). From a common sac-shaped ancestral form with simple ectoderm, simple entoderm, and undifferentiated cells in the intervening mesogloea, representing the type of the Coelentera, both Mesodermalia and Epithelaria have been developed. In the case of the Mesodermalia the cells of the mesogloea became differentiated, and produced the organs, whilst the epithelia remained simple. In the case of the Epithelaria the cells of the mesogloea remained unchanged and the organs were produced by the epithelia. I regard this as the principal difference dividing the two groups, and have therefore established the term Epithelaria in contradistinction to Mesodermalia (I. c). Having thus described the points of distinction, it remains that we should ascertain their phylogenetic value. There are only two alternatives with regard to the value we may attach to the Sponges as a group. Either we must assume that within tbe Grade Coelentera the Phylum Mesodermalia and the Phylum Epithelaria should be distinguished ; or we may say that there is only one phylum in the Grade Coelentera, namely* the Phylum Coelentera, aud that this should be divided into the two Subphyla Mesodermalia and Epithelaria. It is evident that it comes much to the same thing. In this matter I adopt F.E. Schulze's opinion (1369), and consider the Sponges a separate phylum. The result of this critical examination is given in the accompanying tabular view. If we express this arrangement in the usual manner, we have :- |
