1886.] POSITION AND CLASSIFICATION OF SPONGES, 5G7 



boid cells met with iu the Sponges are invariably modified cells of the 

 mesogloea. This is particularly striking and important in the case of 

 the muscular and sensitive elements. 



The Epithelaria, on the other hand, have a mesoglcea 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 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 

 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 mesogloea. Single muscular cells 

 are never surrounded on all sides by the mesogloea. Solid bundles 

 of muscular cells do not occur. Exceptionally nettle-cells {Cram- 

 bessa) may be found in the mesogloea, 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 meso- 

 gloea, representing the type of the Coelentera, both Mesodermalia 

 and Epithelaria have been developed. In the case of the Mesodermalia 

 the cells of the mesoglcea became differentiated, and produced the 

 organs, whilst the epithelia remained simple. In the case of the 

 Epithelaria the cells of the mesoglcea 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 {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 the Grade Coelentera the 

 Phylum Mesodermaha and the Phylum Epithelaria should be distin- 

 guished ; or we may say that there is only one phylu n in the Grade 

 Coelentera, namely the Phylum Coelentera, 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 : — 



