520 POLYPI. 



In the fourth tribe the animal rind or bark encloses a mere fleshy 

 substance without an axis either osseous or horny. In 



Alcyonitjm, Lin. 



As in the Pennatuls, we observe Polypi with eight denticulated arms, and 

 intestines prolonged into the common mass of the ovaries: but this mass is 

 not supported by an osseous axis; it is always fixed to the body; and where 

 it is drawn out into trunks and branches, nothing is found internally, but a 

 gelatinous substance traversed by numerous canals surroimded with fibrous 

 membranes. The bark is harder and excavated by cells, into which the 

 Polypi withdraw more or less entirely. 



After the Alcyonia are also placed the 



Spongia, Lin. 

 Or Sponges; marine, fibrous bodies, whose only sensible portion appears to 

 be a sort of tenuous gelatine which dries oil", scarcely leaving a trace of it, 

 and in which neither Polypi nor other moving parts have yet been disco- 

 vered. Living Sponges are said to exhibit a sort of tremulousness or con- 

 traction wh en they are touched; It is also affirmed that the pores, with their 

 superficies, are perforated, and present a sort of palpitation; the existence 

 of these motions, however, is contested by M. Grant. 



Sponges assume innumerable shapes, each according to its species, and 

 resemble shrubs, horns, vases, tubes, globes, fans, &c. 



Every one knows the S. officinalis, or common Sponge, which is found 

 in large brown masses, formed of extremely fine, flexible, and elastic fibres, 

 perforated with numerous pores and little irregular canals, all of which in- 

 tercommunicate. 



CLASS V. 



INFUSORIA. 



Naturalists usually close the catalogue of the animal kingdom 

 with beings so extremely minute as to be invisible to the naked eye, 

 and which have only been discovered since the invention of the mi- 

 croscope has unveiled to us, as it were, a new world. Most of 

 them present a gelatinous body of the greatest simplicity, and for 

 these, this is undoubtedly the situation; but authors have placed 



