FOR 



[ 208 ] 



FOR 



sometimes resembles a brick which has 

 undergone a slight vitrification. Its 

 fracture is imperfectly conchoidal or 

 uneven, more or less glistening, and 

 often has the aspect of certain porce- 

 lains. It is opaque, very brittle, and less 

 hard than quartz. Before the blow-pipe 

 it melts into a black scoria. An analysis 

 of it yielded silex 60'75, alumine 27'25, 

 potash 3-66, magnesia 3'00, oxide of 

 iron 2-50. It is most likely an alteration 

 of some variety of argillaceous slate by 

 pseudo-volcanic fires : it does not consti- 

 tute a distinct species. It is found in 

 large masses near the pitch-lake in Trini- 

 dad, and occurs usually in the vicinity of 

 coal mines. Cleaveland. 

 PORI'FERA. )from porus, a pore, and 

 PORI'PHERA. $ fero, to bear.) A class 

 of animals belonging to Cyclo-Neura, 

 or Radiata. Poriphera constitute the 

 second lowest class of animals, com- 

 ing between Polypiphera and Polygas- 

 trica. They form the various species 

 of sponge which are met with in such 

 multitudes on every rocky coast of the 

 ocean, from the shores of Greenland to 

 those of Australia. On the shores of 

 the sea, says Professor Grant, when the 

 tide has retired, rocks and other marine 

 substances may be seen covered with 

 a layer of a soft spongy substance, of va- 

 rious forms, colours, and consistence, 

 sometimes hanging in branches from the 

 cliffs which are covered by the sea, 

 sometimes covering the surface of marine 

 animals. This layer consists of beings 

 which possess an organization extremely 

 simple. Their surface is porous ; those 

 pores lead to canals which ramify through 

 all parts of their texture ; and those 

 canals anastomosing into larger and 

 larger trunks, lead, again, to orifices on 

 the surface, from which there issue con- 

 stant streams of water. The poriferous 

 animals present various and remarkable 

 forms in the skeleton ; and the simple 

 gelatinous body of the animal is sup- 

 ported by a skeleton composed of dif- 

 ferent kinds of earth : in one group the 

 earth is silica ; in another it is the car- 

 bonate of lime ; in another it is a horny 

 substance. The skeleton, thus composed, 

 has been called the axis of the animal. 

 The material of which the fleshy portion 

 is composed is of so tender and gelatinous 

 a nature, that the slightest pressure is 

 sufficient to tear it asunder, and allow 

 the fluid parts to escape ; and the whole 

 soon melts away into a thin oily liquid. 



The surface of a living sponge presents 

 two kinds of orifices ; the larger of a 

 rounded shape with raised margins, which 

 form projecting papillae ; the smaller, mi- 

 nute and numerous, constituting the pores 



of the sponge. It was long the received 

 opinion that the superficial layer of gela- 

 tinous substance possessed a considerable 

 degree of contractile power, nor was it till 

 Prof. Grant clearly demonstrated that the 

 sponge does not possess any such property 

 that the illusion was dissipated. The 

 porifera present a digestive system, which, 

 "by its form and simplicity, approaches the 

 nearest to that of plants. The cellular 

 tissue of their body is permeated in all 

 directions by anastomozing and ramifying 

 canals, which begin by minute superficial 

 pores closely distributed over every part, 

 and terminate in larger orifices variously 

 placed according to the form of the entire 

 animal. The pores are provided with a 

 gelatinousjnetwork and projecting spicula, 

 to protect them from the larger animal- 

 cules and floating particles The internal 

 canals, like the venous system, leading 

 from capillaries to trunks, are bounded 

 by a more condensed portion of the general 

 cellular substance of the body, and are 

 incessantly traversed by streams of water, 

 passing inwards through the minute pores, 

 and discharged through the larger orifices 

 or vents, but|no polypi or cilia have been 

 discovered in those parts, although from 

 analogy we might consider them neces- 

 sary. From the incessant streams that 

 are conveyed through the bodies of these 

 animals, it appears that all parts of these 

 interior perforations, as well as the general 

 external surface of this cellular structure, 

 serve for the conveyance of nutritious 

 matter into the interior substance of the 

 body. On watching the streams of water 

 which issue from the foecal orifices, there 

 may be seen minute flocculent particles 

 that are incessantly detached and thrown 

 out, which appear as if they were the 

 residue of digestion, or pellicles excreted 

 from the bodv, and thrown off from the 

 surface of internal canals. 



No nervous filaments have been de- 

 tected in the soft gelatinous bodies of 

 poriphera. Their ciliated gemmules, 

 however, are endowed with remarkable 

 living properties, and powers of sponta- 

 neous motion. They have an evident 

 object in their motions ; they can accele- 

 rate, retard, or cease, at pleasure, the 

 vibrations of their cilia ; they can change 

 the direction of their course in ifoe water, 

 perceive each other's vicinity, revolve 

 round each other, distinguish the most 

 suitable place for the fixing of each spe- 

 cies, or bound forward suddenly from a 

 state of rest. They appear in this state 

 of freedom to be sensible to light, and to 

 shun it. 



Although sponges, or poriferous ani- 

 mals, are permanently attached to rocks, 

 and other solid bodies in the ocean, and 



