136 Dr Grant's Observations on the Structure 



tudes of the two hemispheres ; indeed the geographical distribu- 

 tion of the species cannot be satisfactorily ascertained till their 

 characters are better described and defined. This animal, how- 

 ever, seems eminently calculated for an extensive distribution, 

 from the remarkable simplicity of its structure, and the few 

 elements required for its subsistence. Its inertness, its soft ge- 

 latinous structure, its want of organs for seizing prey, the 

 incessant currents through its body, and the growth of its 

 ova, when nourished only with sea-water, shew that it sub- 

 sists either on the elements of that fluid, or on the minute 

 particles of organic matter suspended in it. Its canals pre- 

 sent the first rudiments of an internal stomach ; by these sim- 

 ple organs it extracts a mass of gelatinous matter from the wa- 

 ters of the ocean, and organises it for the digestive organs of ani- 

 mals higher in the scale. Its interior affords a domicil and a 

 magazine of food for myriads of minute marine animals. It ex- 

 tracts silicious matter from the ocean, and precipitates it in re- 

 gular and beautiful crystalline forms. It precipitates, in the 

 form of an insoluble carbonate, the calcareous matter continually 

 poured by rivers into the bed of the ocean in a soluble state ; it 

 thus assists in purifying the vast abyss of a corrosive ingredient, 

 and prepares it for the maintenance of the various tribes of ver- 

 tebral inhabitants that people its boundless expanse. And it 

 has probably aided in the formation of silicious and calcareous 

 rocks. 



I have now given a brief outline of the natural history of the 

 Sponge as a genus, and stated the laws which regulate its external 

 form, in so far as I have been able to observe the living characters 

 and habits of the species in the Frith of Forth. I have endeavoured 

 to trace to their sources the discoveries which have been succes- 

 sively made in its structure and economy^ and have shewn, that 

 the true nature of this singular being, and the uses of all its 

 parts, were as well known to the ancient Greeks as to the natu- 

 ralists of modern Europe, — that the description of it given by 

 Aristotle is more correct and complete than that of Lamarck. 

 I have detailed a series of experiments to determine the uses of 

 the pores, canals and orifices ; and have shewn, that the inces- 

 sant currents through these passages, which are subservient to 



