P R 



[ 364 



P R 



larger trunks, lead, again, to ori- 

 fices on the surface, from which 

 there issues constant streams of 

 water. The poriferous animals 

 present various and remarkable 

 forms in the skeleton ; and the 

 simple gelatinous body of the 

 animal is supported by a skeleton 

 composed of different kinds of 

 earth : in one group the earth is 

 silica; in another it is the carbonate 

 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 com- 

 posed 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 



E resents two kinds of orifices ; the 

 irger of a rounded shape with 

 raised margins, which form pro- 

 jecting papillae ; the smaller, mi- 

 nute and numerous, constituting 

 the pores of the sponge. TJie 

 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 anastomosing 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 gelatinous network and pro- 

 jecting spicula, to protect them 

 from the larger animalcules 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 sub- 

 stance of the body, and are inces- 

 santly 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 necessary. From the inces- 

 sant 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 inces- 

 santly detached and thrown out, 

 which appear as if they were the 

 residue of digestion, or pellicles 

 excreted from the body, and thrown 

 off from the surface of internal 

 canals. 



No nervous filaments have been 

 detected in the soft gelatinous 

 bodies of poriphera. Their ciliated 

 gemmules, however, are endowed 

 with remarkable living properties, 

 and powers of spontaneous motion. 

 They have an evident object in 

 their motions ; they can accelerate, 

 retard, or cease, at pleasure, the 

 vibrations of their cilia ; they can 

 change the direction of their course 

 in the water, perceive each other's 

 vicinity, revolve round each other, 

 distinguish the most suitable place 

 for the fixing of each species, 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 

 animals, are permanently attached 

 to rocks, and other solid bodies in 

 the ocean, and are consequently 

 destined to an existence as com- 

 pletely stationary as that of plants, 

 yet such is not the condition of the 

 earlier, and more transitory stages 

 of their development. On the 



