1886. ] POSITION AND CLASSIFICATION OF SPONGES, 567 
boid cells met with in the Sponges are invariably modified cells of the 
mesogleea. This is particularly striking and important in the case of 
the muscular and sensitive elements. 
The Epithelaria, on the other hand, have a mesoglea the cells of 
which remain more or less amceboid 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 sur- 
face of the mesoglcea or supporting lamella. 
By a process of folding and subsequent coalescing of the fold- 
margins, bundles of muscular cells may become immersed in the 
mesogloea, and so form a mesodermal structure, which, however, 
must be considered a secondary mesoderm, as compared to the 
primary mesoderm represented by the mesogloea 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 mesoglcea. 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 Meduse). 
From a common sac-shaped ancestral form with simple ectoderm, 
simple entoderm, and undifferentiated cells in the intervening meso- 
gloea, representing the type of the Ccelentera, 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 Mesoder- 
malia (J. 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 the Grade Ccelentera the 
Phylum Mesodermalia and the Phylum Epithelaria should be distin- 
guished ; or we may say that there is only one phylunin the Grade 
Ceelentera, namely the Phylum Ccelentera, and 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 :-— 
