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Show 1886.] POSITION A N D CLASSIFICATION O F S P O N G E S . 571 the other Sponges. Grant (526) was the first to point this out, and he accordingly divided the Sponges into Calcarea and Non- Calcarea. Vosmaer (1550) agrees in this point with Grant and uses his terms. I have also (888) adopted the same view. In this paper I intend to alter the term Non-Calcarea, which is misleading, inasmuch as it might be interpreted as meaning that the group so named consisted of very heterogeneous elements, coinciding with each other only in one, and that a negative character. This is not the case. I divide the Classis Spongise accordingly into two Subclasses, I. Calcarea, and II. Silicea. The point of distinction between these two Subclasses is, that all the Calcarea have a skeleton composed of spicules consisting chiefly of carbonate of lime. All the other Sponges, which I comprise under the heading Silicea, either have a skeleton composed of siliceous spicules or have been derived phylogenetically from siliceous Sponges, and have only recently lost their spicules or replaced them with a horny support. O. Schmidt (1305) and also myself (870) were inclined to think that some of the siliceous Sponges had descended from horny ones. I have, however, since abandoned this view (901), and consider that the opposite direction of development, which Vosmaer (1558) advocates, is the correct one. W e have accordingly :- Classis SPONGLE. Skeleton composed chiefly of Skeleton originally composed carbonate of lime. of siliceous spicules. I. Subclassis C A L C A R E A . II. Subclassis SILICEA. As mentioned above, in the critical introduction to this chapter, I have nothing to add to m y system of Calcareous Sponges (888) published some time ago, and I adopt it unchanged in this paper. The Calcarea are a very much smaller group than the Silicea. In this Subclass we only distinguish one Order, the Calcispongiae (Blainville); whilst the Silicea must be divided into several Orders, and it is here that we meet with the greatest difficulty in ascertaining the true relationship of the different forms. There are no transitions between the two subclasses. In examining the structure of a great number of Sponges belonging to this second group, the subclass Silicea, I found that they can be arranged in three Groups, which will appear as Orders in m y system. These are the Hexactinellida, the Chondrospongiae, and the Cornacuspongise. These groups are fairly distinct, and transitional forms connecting them are rare. The Sponges of these Orders are descended from siliceous Sponges, and show the same tendency of development within each group. In the Hexactinellida we invariably meet with a skeleton composed of triaxial spicules; these are often attached to each other by a siliceous cement which greatly strengthens the structure. All authors agree that the Hexactinellida form a well-defined group. The remaining Silicea, however, are a very mixed lot, and before Vosmaer, no satisfactory arrangement of them had been arrived |
