PORIFERA 97 



small oscula and large excurrent canals, the distinction of indi- 

 viduals from colonies is likewise difficult. 



The sponge is not a colony of protozoons, but an individual, 

 for not until an osculum is formed can it feed and reproduce. 



The skeleton consists of calcareous or siliceous spicules or of 

 horny fibers, all developed in the mesogloea. In the develop- 

 ment of either calcareous or siliceous spicules there is first 

 formed an organic thread at the center of the cell ; around this 

 axial thread the mineral matter is deposited. The calcareous 

 spicules, of almost pure calcium carbonate, are deposited in a 

 crystalline condition, each spicule behaving optically like a 

 single crystal. The siliceous spicules are composed of colloidal 

 silica which is deposited in concentric lamellae alternating with 

 organic lamellae. The spicules of the Hexactinellida, when 

 united, are joined by a secondary deposit of silica, but those of 

 the Tetractinellida and Monactinellida are at times united 

 by spongin, at others the spicules are enveloped in spongin, as 

 in Chalina ; finally in the Ceratospongida the spicules disappear 

 and spongin alone remains, as is seen in the common bath sponge. 

 Sponges are classified according to this skeleton. 



The Myxospongida, with no skeletal elements, and the 

 Ceratospongida, with skeleton of perishable fibers of spongin, 

 can seldom be preserved as fossils. Practically all fossil sponges, 

 therefore, belong to the orders secreting siliceous or calcareous 

 spicules. In these orders fossils may be preserved in two states. 

 f If the skeletal framework is sufficiently firm to hold together 

 after the death of the animal, and consequent disappearance 

 of the soft parts, it may be preserved in its entirety, as for 

 example in Prismodictya and Astylospongia. If, however, 

 this framework is composed of spicules so loosely bound together 

 as to fall apart upon the disappearance of the soft parts, the 

 scattered spicules alone will be preserved, as in Lyssacina, Cliona 

 and Renieria. 



During fossilization the composition of a sponge skeleton 

 may be greatly modified. In those forms with siliceous spicules 



