INTRODUCTION. 



A SPONGE, in the ordinary acceptation of the word, is the internal framework or 

 skeleton of an animal whose soft vital parts are composed of cellular protoplasm. 

 This protoplasm in the living sponge is differentiated into two layers — an inner of 

 ciliated cells, and an outer layer, named the syncytium, in which tlie cell-structure in 

 the ordinary condition is not recognizable. It is in this syncytium or exoderm that 

 the skeleton is secreted. In one small group of existing sponges the skeleton is 

 wanting and the entire animal consists of fleshy sarcode ; but the rest of the class 

 secrete skeletons of very various composition and form. In the division to which 

 the ordinary sponge of commerce belongs, the skeleton is composed of elastic fibres of 

 a horny character, which anastomose together and build up those porous structures 

 with which we are all so familiar. In another group the horny fibres have a central 

 core composed of minute siliceous spicules usually secreted by the animal, though 

 occasionally the central portion of the fibre is filled with grains of sand and other 

 foreign particles which the sponge has selected from its surroundings. In yet other 

 sponges the skeleton consists almost exclusively of minute siliceous or calcareous 

 bodies, more or less intimately united together to form a resistant framework of a 

 porous character, which is generally traversed by numerous canals. The minute 

 mineral particles or spicules of the skeleton vary exceedingly in their form and 

 arrangement in different groups ; they possess an interior canal, and are built up of 

 exceedingly thin concentric layers of silica or calcite mingled vi'ith organic material. 

 The classification of the Sponges, recent as well as fossil, rests upon the characters 

 of their skeletal structures. The existing forms of the class have been divided into 

 the following orders : — 



1. Myxospongi^, Haeckel. 

 Sponges destitute of a solid skeleton. 



2. CERATOSPONGiiE, Bronn. 

 Sponges with skeletons of horny fibres. 



