OR OSSEOUS SYSTEM. 



in plants with which these poripherous animals are the most 

 nearly allied. 



Several of the animals of this class have the skeleton 

 composed of calcareous spicula which have generally more 

 complex forms than the silicious. They are disposed in the 

 same manner and for the same object through the in- 

 terior cellular substance of the body. They impart a white 

 colour to the whole body of the animals in which they occur. 

 They do not appear to occur along with silicious spicula in 

 the same animal. The skeleton is generally more loose and 

 friable in the calcareous poriphera, and the connecting gluti- 

 nous and cellular matter is less abundant. One of these 

 white friable calcareous poriphera, the leuconia compressa, 

 very common in our seas, is represented in Fig. 3 at 4 in 

 form of a compressed FIG - IIT - 



lengthened sac suspend- 

 ed by its peduncle from 

 any submarine substance 



The pores through 

 which the currents are 

 conveyed into this sack 

 are seen all over the ex- 

 terior surface, as at a a ; 

 the canals are contained 

 within the thickness of 

 its parietes, and the large 

 vents or fecal orifices here 

 open into the interior, as 

 seen where it is broken 

 open at b. The sac being 

 open only at its pendent 

 extremity d, the whole of the inhaled water rushes incessantly 

 out through that general aperture. In the silicious skeletons 

 of this class we find but one form of spiculum for each ani- 

 mal ; but in the calcareous generally two, and one of these 

 has a triradiate form, as represented in Fig. 3 at 1. This 

 triradiate form of calcareous spiculum is accompanied by one 

 of some other form, as by that clavate form of spiculum be- 

 longing to the leuconia compressa, shewn in Fig. 3 at 2 ; the 

 small spicula in Fig. 3 at 3, found in the same animal ap- 



