and Functions of the Sponge. 137 



the nourishment, respiration, and reproduction of the animal, 

 are not produced by the alleged irritability of its axis, nor by 

 the supposed systole and diastole of its apertures, but by cer- 

 tain minute organs disposed over the whole surface of the inter- 

 nal canals. I have described the most striking differences which 

 I have observed in the chemical constitution and microscopical 

 forms of the minute parts composing the skeleton of this ani- 

 mal, in the three great tribes of horny, calcareous, and silicious 

 species, and their beautiful arrangements to maintain the gene- 

 ral form of the zoophyte, and to support and defend its soft 

 parts. I have stated the characteristic properties and appear- 

 ances of the connecting matter of the spicula, the parenchyma.- 

 tous, or general cellular substance of the body, the gelatinous 

 net- works of the pores, and the granular bodies of the internal 

 canals. I have examined the successive changes which the ova 

 undergo from the time of their first appearance in the parenchy- 

 matous substance of the parent till their full development, and 

 their expulsion from the fecal orifices, the causes of the singular 

 spontaneous motions they exhibit, from the time of their expul- 

 sion till their metamorphosis into fixed inert zoophytes, and the 

 progress of their growth in this fixed state, till they attain the 

 perfect form of the parent. And, lastly, I have stated a few 

 observations on their geographical distribution, and their pur- 

 poses in the economy of nature. The uses of the central cavi- 

 ties in the horny fibres, and in the earthy spicula, and the diffe- 

 rent forms of these elementary parts, in all the known species ; 

 — the mode in which the animal imbibes nourishment through 

 the parietes of the internal canals, and the chemical changes 

 produced on the fluid by its transmission through these pas- 

 sages ; — the particular tribes of infusoria and more perfect ani • 

 mals that infest the different species, and depend on them for 

 subsistence, and the applications of the earthy species of tliis 

 animal to useful purposes in the arts, are still unknown. No 

 one has yet excited to action any part of the adult animal, and 

 the moving organs of the currents have never been seen. The 

 mode of generation of this animal, and the structure of its soft 

 parts, have yet been examined only in a very few species. The 

 characters and the geological distribution of its organic remains 

 have yet to be investigated, and probably not a tenth part of 



