1'ORIFERA. 13 



this sense cf the word. The sensitive plant, for example, which 

 droops its leaves upon the slightest touch, would have far greater 

 claims to be considered as being an animal than the sponges of 

 which we are speaking. 



The power of voluntary motion has been appealed to as exclusively 

 belonging to the animal economy : yet, setting aside the spontane- 

 ous movements of some vegetables, the sponge, rooted to the rock, 

 seems absolutely incapable of this function, and the most micro- 

 scopic scrutiny has failed to detect its existence. 



The best definition of an animal, as distinguished from a vege- 

 table, which has as yet been given, is, that whereas the latter fixed 

 in the soil by roots, or immersed perpetually in the fluid from 

 which it derives its nourishment, absorbs by its whole surface the 

 nutriment which it requires ; the animal, being generally in a 

 greater or less degree capable of changing its position, is provided 

 with an internal receptacle for food, or stomachal cavity, from 

 which, after undergoing the process of digestion, the nutritious 

 matter is taken up. But in the case of the sponge no such 

 reservoir is found; and in its place we find only anastomosing 

 canals which permeate the whole body, and convey the circumam- 

 bient medium to all parts of the porous mass. 



The last circumstance which we shall allude to as specially 

 appertaining to the animal kingdom, is derived from the chemical 

 composition of organized bodies. Vegetables contain but a 

 small proportion of azote in their substance, whilst in animals this 

 element exists in considerable abundance, causing their tissues 

 when burned to give out a peculiar odour resembling that of 

 burned horn, and in this particular sponges differ from vegetable 

 matter. 



(16.) The common sponge of commerce is, as every one knows, 

 made up of horny, elastic fibres of great delicacy, united with each 

 other in every possible direction, so as to form innumerable canals, 

 which traverse its substance in all directions. To this structure 

 the sponge owes its useful properties, the resiliency of the fibres com- 

 posing it making them, after compression, return to their former 

 state, and leaving the canals which they form open, to suck up 

 surrounding fluids by capillary attraction. 



The dried sponge is, however, only the skeleton of the living 

 animal : in its original state, before it was withdrawn from its native 

 element, every filament of its substance was coated over with a thin 

 film of glairy semifluid matter, composed of aggregated transparent 



