OR OSSEOUS SYSTEM. 5 



about thirty known genera of polygastric animalcules which 

 possess a firm elastic exterior covering, more or less envelop- 

 ing the body, and analogous to the more solid skeletons of 

 higher classes. 



II. Poriphera. The skeleton of poripherous animals con- 

 sists of separate minute, earthy, crystalline spicula, connected 

 together by a condensed, elastic, cellular substance j or of 

 tubular elastic filaments of a horny consistence. These hard 

 parts are developed internally throughout the whole cellular 

 tissue of the body, and are often protruded externally through 

 the surface, to protect the pores, or the large vents. The 

 earthy spicula in most of these animals are silicious, 

 in many they are calcareous, and, like the horny filaments of 

 other species, they appear to be tubular like many natural 

 crystals, and to have no aperture leading into their internal 

 cavity. The spicula are generally united into fasciculi by an 

 enveloping glutinous, or condensed cellular substance, and by 

 the junction of these fasciculi in various modes, fibres are 

 formed which traverse every part of the body, forming the 

 boundaries of canals and orifices, and giving form and sup- 

 port to the whole of the gelatinous or soft cellular substance 

 of the animal. The forms of these hard parts are different 

 in every distinct species of these animals, and they are con- 

 stant in the same, so that they present useful characters for 

 the distinction of species in this polymorphous class. They 

 are formed from materials due to the vital energies of the 

 animal, and they form normal and necessary parts of its 

 structure, like the solid skeletons of higher animals. In 

 Fig 2 is represented at A the haliclona occulata ; one of these 

 soft animals, with a silicious skeleton. It is represented as 

 alive, suspended from a rock by its spreading branched base 

 of attachment (c,) the currents of water are seen at (a) rush- 

 ing in through the pores, and issuing from the internal 

 canals by the large orifices or vents at (b). The pores, canals, 

 and orifices are seen exposed in the longitudinal section of 

 the same poripherous animal at B. Fibres composed of 

 bundles of spicula generally extend in a longitudinal direc- 

 tion in these animals from the base of attachment to the 

 remotest points of the surface. Smaller transverse fibres of 

 the same composition connect those which are disposed 

 longitudinally, and form the frame-work of the internal 



