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- 
Calearea. 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-Calearea, 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 Spongize accordingly into two Subclasses, I. Cal- 
carea, and ITI. 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 some of the siliceous Sponges had 
descended from horny ones. I have, howey er, 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 SPONGLAS. 
Skeleton composed chiefly of Skeleton originally composed 
carbonate of lime. of siliceous spicules. 
I. Subclassis CaucarEa. II. Subclassis Srzicea. 
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 Calcispongiz 
(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 my system. These are the Hexactinellida, 
the Chondrospongi, and the Cornacuspongiz. 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 
