134 PROTOZOA. 



though doubtless capable of preservation, and though prob- 

 ably often really present in the rocks, can be with difficulty 

 detected, from their minute size, and can hardly be said to 

 be known with certainty except in the Secondary and Ter- 

 tiary deposits. Many fossil Sponges have, it is true, been 

 referred by different observers to the section of the Horny 

 Sponges, but it is now certain that most of these are certainly 

 truly Siliceous Sponges, while the others are equally referable 

 to other groups. 



An exception to the above statement must be made in 

 favour of the aberrant group of living Sponges known as 

 the Clionidce. In all formations, from the Lower Silurian 

 onwards, we meet with shells and corals mined with winding- 

 tunnels or borings, which have a circular cross-section. These 

 tunnels are usually regarded as being the work of Sponges 

 belonging to the living genus Cliona (Vioa), or to forms 

 closely allied to this ; and in many instances this reference is 

 doubtless correct. It must not be forgotten, however, that 

 it is very difficult, or impossible, to distinguish, in the case 

 of fossils, between the borings made by Sponges and those 

 produced by Annelides or by carnivorous Gasteropods. Gei- 

 nitz has also described from the Permian rocks a sponge to 

 which he gives the name of Spongil^sis, and which he 

 regards as being most nearly allied to the living fresh- water 

 sponges (Spongilla). Lastly, we meet with the remains of 

 Sponges, as yet undescribed, in the Lower Silurian rocks of 

 Britain, which show some indications of having been origin- 

 ally horny. Of this nature, perhaps, is the cup-shaped Proto- 

 spongia of Salter ; but the minute structure of this old type 

 is still very imperfectly known. 



2. THE CALCAREOUS SPONGES (Calcispongice). The Sponges 

 included in this group are invariably furnished with a cal- 

 careous skeleton, which, in all the living species, is composed 

 of spicula of lime, usually fusiform or triradiate in shape, and 

 always entirely free and independent of each other. No 

 living member of the Calcispongice, then, is possessed of a 

 continuous skeleton, and the calcareous spicules which con- 

 stitute the sole skeletal elements are microscopic in their 

 dimensions ; so we might have anticipated beforehand that 

 we should find few representatives of this group of Sponges 



