SPONGIDA. 67 



foramina for the emission of pseudopodia, and often provided 

 with spine-like projections. The Polycystina are best known 

 as occurring in the so-called " Infusorial Earth " of Barbadoes. 

 This is a Tertiary deposit, and consists largely of the shells of 

 Polycystina. They have not as yet been detected in any 

 Palaeozoic formation, but they are known to occur in the 

 Mesozoic series. 



III. SPONGIDA. 



The Sponges may be defined as Rhizopoda composed of 

 numerous amxbiform masses of sarcode united into a composite 

 mass, which is traversed by canals opening on the surface, and is 

 almost always supported by an internal skeleton or framework of 

 horny fibres or of calcareous or siliceous spicula. 



The only portion of the Sponges with which the palaeonto- 

 logist is concerned, is the skeleton. Whatever the nature of 

 this skeleton may be, it is so arranged that its parts surround 

 two sets of apertures which open on the surface of the sponge, 

 and which are connected with one another by a system of 

 canals ramifying in its deeper portions. Of the apertures 

 which penetrate the substance of the sponge in every direc- 

 tion, one set consists of large chimney-like openings, which are 

 called " oscula," or " exhalant apertures." There maybe only 

 a single osculum, or many may be present. The other set 

 consists of very much smaller openings, which are always very 

 numerous, and which are termed the " pores," or " inhalant 

 apertures." The pores and oscula are connected by a system 

 of canals excavated in the substance of the sponge, and a con- 

 stant circulation of water can be kept up through the whole 

 mass, the former serving for the incoming currents, the latter 

 for the outgoing. 



The Sponges are divided into three groups according to the 

 nature of the skeleton : i. Keratosa, comprising the ordinary 

 sponges of commerce, in which the skeleton is composed of 

 a horny substance called "keratode;" 2. Calcarea, or Calci- 

 spongia, in which the skeleton is composed of carbonate of 

 lime ; and 3. Silicea, or Vitrea, in which the skeleton is com- 

 posed of flint or silex. 



The Horny Sponges, from the nature of their skeleton, are 

 not certainly known as fossils ; but traces of their past exist- 

 ence are said to have been obtained in the form of the spicules 

 with which the horny skeleton is sometimes furnished. The 

 Calcareous and Siliceous Sponges are both well represented in 

 a fossil condition, though the true nature of some of the more 



