THE SIMPLEST MANY-CELLED ANIMALS 



321 



These are the skeletons of colonies of animals, the flesh hav- 

 ing been removed when preparing the sponges for market. 

 Besides these common sponges, there are certain sponge- 

 animals which have skeletons made up of glass spicules, and 

 others which form 

 their skeletons of lime 

 (calcareous) spicules. 

 These various mate- 

 rials found in sponge- 

 animals are secreted 

 by living cells, just 

 as the cells of higher 

 animals secrete bone. 

 The general plan of 

 one of the simplest 

 sponge-animals is 

 shown in Fig. 96. On 

 the outside are nu- 

 merous pores. There 

 is a central cavity, 

 which has an opening 

 at the top. This cen- 

 tral cavity is con- 

 nected by small tubes 

 (see the figure) with 

 the pores on the sur- 

 face of the body, and 

 water flows in through 

 these tubes or canals 

 and out at the top of 

 the central cavity. As shown in the figure, the canals starting 

 from the external pores do not run quite to the central cavity, 

 and other canals starting from the central cavity extend al- 

 most to the surface. Between these inner and outer canals 

 are small openings ; so that water entering the outer canal can 



Y 



FIG. 96. A simple sponge, the branch at the 

 right cut to show central cavity, o, opening 

 of central cavity; ip, inhalent pores. The 

 young branch at left has been formed by 

 budding. (From Parker and HaswelL) 



