1886.] POSITION AND CLASSIFICATION OF SPONGES. 571 



the other Sponges. Grant (526) was the first to point this out, 

 and he accordingly divided the Sponges into Calcarea and Non- 

 Ciilcarea. 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. Cal- 

 carea, 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 sili- 

 ceous 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 soine of the siliceous Sponges had 

 descended from horny ones. I have, however, since abandoned this 

 view (901), and consider that the opposite direction of develop- 

 ment, which Vosmaer (1558) advocates, is the correct one. 



We have accordingly : — 



Classis SPONGIiE. 

 Skeleton composed chiefly of Skeleton originally composed 



carbonate of lime. of siliceous spicules. 



I. Subclassis Calcarea. II. Subclassis Silicea. 



As mentioned above, in the critical introduction to this chapter, I 

 have nothing to add to my 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 Caicispongiae 

 (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 diiFerent 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 my system. These are the Hexactinellida, 

 the ChondrospongitE, and the CornacuspongiiB. 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 gnatly 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 



