PROTOZOA. 177 



formed ; but it is not certain whether each single cell in these instances has 

 not arisen from the fusion of two others. (Micro. Diet.) 



SPONGIA (Sponge), Linnaeus. Sub-class. Spongidae characterized as fol- 

 lows : Fixed, aquatic, polymorphous animals ; inhaling and imbibing the 

 surrounding elements through numerous contractile pores situated on the 

 external surface ; conveying it through internal canals or cavities, and eject- 

 ing it through appropriate orifices ; having an external flexible or inflexible 

 skeleton. (Bowerbank.)* 



Order Keratosa. 



Spongia is distinguished as follows : Skeleton kerato-fibrous ; fibre solid, 

 cylindrical, aspiculous ; rete unsymmetrical. 



S. cfficinalis, Linnaeus. 



The Spongice Ciliatoe are held by Prof. H. J. Clark to belong to the same 

 family with the Infusoria Flagellata, the connecting form being secured in / I 

 the genera Codosiga and Leucosolenia. 



* The other divisions of the Spongia are : 



ffalifiaracidse. Most elementary forms, in which a system of water cavities has not 

 been demonstrated. 



ClionidsR, or sponges, which bore into shells by means of their siliceous spicula. In- 

 dications of their existence extend down to the Silurian strata. 



Petrospongidss, A group of organisms known only in the fossil state and abound- 

 ing in the chalk. They can only be said to be probably sponges. 



Tethyidse. Undoubted sponges, but of very hard texture, and furnished with very 

 long spicula which radiate from the centre of each organism to its circumference. 

 (Huxley.) 



12 



