CHAPTER III 



osc. 



spi.J 



THE SUBKINGDOM PARAZOA (PORIFERA) 



Multicellular organisms ; invariably sessile and aquatic ; with a single 

 cavity in the body, lined in part or almost wholly by collared flagellate 

 cells; with numerous pores in the body wall through which water 

 passes in, and one or more larger openings through which it passes 

 out; and generally with a skeleton, calcareous, siliceous, or horny. 



The members of this phylum are the sponges. 



The simplest sponge is a little creature, 

 known as the Olynthus (Fig. 96), which 

 is found only as a fleeting stage in the 

 development of a few of those members 

 of the group which possess calcareous 

 skeletons ; but the bodies of all sponges 

 may be regarded as derived from it, 

 even though it may not appear as a stage 

 in their life history. It is a hollow vase, 

 perforated by many^ore^, and having at 

 the summit a single large opening, the 

 osculum. Through the pores water con- 

 stantly enters it, to pass out through the 

 osculum. |ierein it and its kind diflPer 

 from all the Metazoa, using the principal 

 opening not for intaking — as a mouth — 

 but for casting out. ^he wall (Fig. 97) of 

 the vase consists of two layers, (a) a 

 gastral layer ^ composed of collared flagel- 

 late cells resembling the Choanoflagellata 

 (p. 59) and known as choanocytes, stand- 

 ing side by side but not touching, which 

 lines the internal cavity or paragaster 

 except for a short distance within the 

 rim ; and {b) a dermal layer ^ which makes 

 up the greater part of the thickness of 

 the wall and is turned in a little way at 

 the rim. This layer again consists of 



two parts, (i) a covering layer of flattened cells, known as pinacocytes, 

 rather like those of a pavement epithelium, but with the power of 

 changing their shape ; and (ii) the skeletogenous layer, between the 

 covering layer and the gastral layer. The skeletogenous layer consists 



Fig. 96. The Olyfithus of a 

 simple calcareous sponge, with 

 part of the wall cut away to 

 expose the paragaster. osc. os- 

 culum; po. pore; spi. spicule. 



