44 BRITISH FOSSIL SPONGES. 
GENERAL CHARACTERS. 
Sponges may be defined generally as animals with bodies of very variable form 
and size, consisting principally of a soft fleshy mass enclosed in a delicate skin. 
The body is penetrated by a system of canals and minute chambers which commu- 
nicate with the exterior by larger, and numerous smaller, apertures. With few 
exceptions they secrete a skeleton, either of horny fibres or of siliceous or calcareous 
spicules, 
Only the skeletal structures of the Sponge are preserved in the fossil state, 
but the true significance of these can only be properly understood by a considera- 
tion of the soft, vital portions of the organism as shown in existing Sponges. There 
is every reason to believe, from the substantial identity of the skeleton in recent 
and fossil forms, that the soft structures of the fossil forms were also essentially 
similar to those of the recent animals. 
In all recent Sponges the free outer surface of the Sponge, as well as the 
interior lacune, and the canals leading from the surface to the ciliated chambers, 
are lined by a delicate membrane, consisting of a single layer of flattened, polygonal, 
nucleated cells forming an epithelium. ‘This is regarded as proceeding from the 
ectoderm of the larva, and therefore styled the ectoderm’ (Schulze). A similar 
layer of epithelial cells lines the canals leading from the ciliated chambers to the 
exterior, and this is regarded as of entodermal origin. The ciliated chambers or 
sacs are lined by cells of sub-cylindrical form, each furnished with a slight project- 
ing collar and a slender flexible cilium or flagellum, also belonging to the entoderm. 
The surface epithelium is penetrated by numerous minute pores, either disposed 
singly or grouped together, which open into the lacunar spaces or into canals which 
convey the water into the interior of the Sponge, and these pores are usually sur- 
rounded by contractile fibres, by which they can be closed and opened. It is also 
penetrated by larger apertures, the vents, or oscules, which are the terminations 
of the canals conveying the water to the exterior, after it has passed through the 
ciliated chambers. These apertures may be either scattered over the surface of the 
Sponge, or grouped round a large central cavity, the cloaca, opening to the exterior. 
The greater portion of the body-substance in living Sponges belongs to the 
mesoderm, and consists of a soft gelatinous, or, in some cases, cartilaginous ground- 
mass of connective tissue, in which are nuclei, granular particles, contractile fibres, 
and various other forms of cells, possessing different functions. Some of these 
cells are amceboid, others spindly or stellate, whilst others are ova and sperm-sacs. * 
1 “On the Structure and Arrangement of the Soft Parts in Hwplectella aspergillum,” ‘Trans. 
Roy. Soe. Edinb.,’ vol. xxix, p. 669, 1880. 
