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 lacunas, 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 1 (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 amoeboid, others spindly or stellate, whilst others are ova and sperm-sacs. 



1 " On the Structure and Arrangement of the Soft Parts in Euplectella aspergillum" ' Trans. 

 Eoy. Soc. Edinb.,' vol. xxix, p. 669, 1S80. 



