50 BRITISH FOSSIL SPONGES. 
In many fossil Sponges the cloaca is altogether absent, or it is merely repre- 
sented by a slight shallow depression at the summit of the Sponge. Thus, in 
extended platter or fan-shaped Sponges no cloaca is present, and the excurrent 
canals usually terminate on the upper or inner surface of the sponge-wall; though 
occasionally they appear on the under or outer surface, and in some instances even 
on both surfaces of the wall. In other Sponges of cylindrical or conical form, in 
which the cloacal cavity is not present, the excurrent canals generally extend 
longitudinally throughout the length of the Sponge, and the vents open at the 
rounded or truncate summit. As arule, the difference in the size of the vents and 
of the ostia or incurrent-canal apertures, is sufficient to determine their respective 
characters ; but in some Sponges the difference is too slight to enable them to be 
separately distinguished. 
In no instance, so far as I am aware, is the presence of ciliated chambers 
indicated in the skeleton of any fossil Sponge, owing no doubt to their minute 
dimensions. 
Canal-system in Fossil Monactinellid and Tetractinellid Sponges.—So few of 
these Sponges are preserved in an entire form in the fossil state that very little 
has been ascertained respecting their canal-structures. In the fossil genera of 
Pachastrella, Opetionella, and Scoliorhaphis, there are only irregular interspaces 
between the spicular tissues, and no definite canals; radial canals are present in 
Vlimacospongia ; and in fossil Cliona the circulation appears to have been similar to 
that of the recent species of the genus, and the vents opened at the surface through 
the circular apertures made in the shell inhabited by the Sponge. 
Canal-system of Fossil Lithistid Sponges.—In this group Zittel’ enumerates 
six different modifications of the water-circulation. 
(1) That in which a special canal-system is altogether wanting. This, of 
course, means that the canal-system must have been disposed in the irregular 
interspaces of the skeletal mesh, since there are no indications of it in the skeleton 
itself. Sponges in which this condition occurs have no cloaca, and no special 
vents ; as examples may be mentioned some species of Platychonia and Holodictyon. 
(2) The sponge-wall is penetrated more or less deeply by branching canals of 
varying size, which open at the surface, the larger being the vents of the excurrent 
canals, and the smaller the apertures of the incurrent canals. The vents usually 
open on the upper surface of the wall, and the incurrent-canal apertures on the 
under surface. he canal-apertures in some instances are equal in size on both 
surfaces of the walls, and the vents cannot then be distinguished from the incur- 
rent canal-apertures. Sometimes the canals enter the wall obliquely. In certain 
instances the excurrent canals are nearly horizontal, and have a stellate arrange- 
ment round the vent. Sponges with this system are platter- or open cup-shaped, 
' «Studien,’ ii Abtheil., p. 75. 
