CHAPTER III 



spi.2 



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, osc. 



known as the Olynthus (Fig. 97), 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 pores, 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. Herein it and its kind difl^er 

 from all the Metazoa, using the principal 

 opening not for intaking — as a mouth — ■ 

 but for casting out. The wall (Fig. 98) of 

 the vase consists of two layers, (a) a 

 gastral layer, composed of collared flagel- 

 late cells resembling the Choanoflagellata 

 (p. 65) 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 Fig. 97. The Olynthus of a 

 rim; and (b) a dermal layer, which makes simple calcareous sponge, with 

 1 ^ ' r 1 1 • 1 r part of the wall cut away to 



up the greater part of the thickness of ^^p^^^ ^^e paragaster. osc. os- 



the wall and is turned in a little way at culum ; po. pore ; spi. spicule, 

 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 



