SPONGES. 113 



closely examined, resembling no vegetable in organi- 

 zation, is found the family of Sponges, a considerable 

 number of which inhabits the shores of the British 

 Islands. Dr. Johnston * enumerates fifty-six species, 

 which he groups under nine genera, distinguished from 

 one another by characters derived from differences in 

 the structure and mineral composition of the skeleton. 

 The outward forms of Sponges are exceedingly sport- 

 ive, and even the same species, at different periods 

 of its life, or under the influence of different circum- 

 stances, often exhibits an outward aspect of very op- 

 posite character. Some are, indeed, tolerably constant 

 in form, especially the branching species ; but the ma- 

 jority are shapeless, or assume a form depending in 

 great measure on the objects in connection with them. 

 It thus becomes necessary, in studying the Sponges, to 

 acquaint ourselves intimately with the exact structure 

 of the skeleton. The spongy body is of the simplest 

 nature ; it consists of a horny or sometimes stony net- 

 work, composed of innumerable interlacing fibres, con- 

 nected together and inosculating, till a porous mass, full 

 of holes and passages, is the result. This is the skele- 

 ton, and such is seen in the common Sponges in every- 

 day use. When the creature is alive, every portion of 

 the horny fibre is coated over with a semifluid slimy 

 matter, like a half-consistent jelly, seemingly inert and 

 unorganized, and yet the seat of whatever life the 

 Sponge contains. It is by this slime, which may be 

 pressed out with the finger, that the network is depo- 



* " History of the British Sponges and Corallines, by G. Johnston, 

 M.D." 



