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Show 572 DR. R. VON LENDENFELD ON THE SYSTEMATIC [Dec. 21, at. The one I propose in this paper, which is similar to that of Vosmaer, is certainly very far from being as perfect as I would like to make it; but I think that at all events it is much more likely to express the relationship of Sponges in a correct manner than any other existing arrangement. The subclass Silicea minus the order Hexactinellida comprises the Sponges with a skeleton composed of tetraxial spicules (the Tetrac-tinellida and Lithistidse of Marshall and other authors), the Sponges with monaxial spicules (the Monactinellida of Zittel and other authors), the Sponges with a horny skeleton and without spicules in the supporting skeleton (the Ceraospongiae or Keratosa of many authors), and, finally, the Sponges without any supporting skeleton at all (the Myxospongiae of Hackel). At first sight all these forms appear connected with each other in every direction by transitional forms to such an extent that it seems hopeless to bring order into this chaotic mass. A careful investigation of many forms shows that all the familiar groups Tetracti-nellida, Lithistidae, Monactinellida, Ceraospongiae, and Myxospongiae run into each other at every point. If one, however, for years endeavours to find some constancy in the varying characters of any chaotic mass of this kind, he at last generally arrives at an ideawhicb seems clear enough when once grasped. And then one only wonders how it was that it had not been conceived a long time ago. So it was also in this case. I found that all these Sponges could be very naturally divided into two Orders-the above-mentioned Chondro-spongiae and Cornacuspongiae, the first of which comprises the Lithistids, Tetractinellida, and portions of the Monactinellida, together with most Myxospongiae ; whilst the second contains all the Ceraospongiae, and the remainder of the Monactinellids and Myxospongiae. W e find that the ground-substance, the mesodermal intercellular substance or Mesogloea, as it is variously termed, is more or less hard and cartilage-like in the Chondrospongiae, and that in these the spicules remain isolated. The spicules are either tetraxon or tylostyles, less frequently styles. The monaxon spicules are monact, thereby indicating their closer affinity with the tetraxon spicules. The necessary toughness is given to these Sponges not by a cementing of the spicules, but by a hardening of the ground-substance. In some the spicules disappear altogether, as in Oscarella, which is an aske-letous form of Plakina, and in Chondrosia, which is an askeletous sponge belonging to the Tethya group. In the Cornacuspongiae, on the other hand, there is no tendency towards a hardening of the ground-substance discernible. The ground-substance remains soft and gelatinous, and the necessary toughness is given to the sponge by the formation of a substance not found in other Sponges, which cements the spicules together. This substance is chemically and physically comparable to silk or horn, and is known as Spongin. The spongin may become very voluminous and the spicules scarce and small. They may finally disappear altogether, and then |
