316 



ANIMALS 



FIG. 258. A Sim- 

 ple Sponge. 



creature dies. The accumulations of coral rock limestone taken 

 from sea water in past ages are enormous in extent (cf. 132). 



Sponges (Fig. 258) are many-celled animals less 

 well developed than hydras. The individual creature 

 has two cell layers separated by a gelatinous layer. 

 The gelatinous layer produces the tough, horny mate- 

 rial that we call "sponge." The animal is very soft, 

 and the horny structure keeps its body cavity open. 

 The cells have cilia, which produce incoming water 

 currents bearing food. The outer openings of the 

 body cavities form the tiny pores of the "sponge." 

 Our sponges therefore represent colonies of the 

 sponge animal with the horny material joined in a 

 large mass. Some sponges form skeletons of lime- 

 stone or of silica (the substance of powdered quartz), instead of horny 

 structures. 



335. Starfishes. One of the interesting pastimes at 

 the seashore is watching the movements and other 

 activities of the starfishes (Fig. 259). The starfishes 

 belong in a class with sea-urchins, 

 sea-cucumbers, etc. This class is 

 more highly developed than 

 sponges and hydras. 



The grown starfish is arranged 

 on a plan of 5. The central region 

 is round, and the "points of the 

 star," or "arms," branch out from 

 it. The covering of the animal 

 contains hard limestone knobs with 

 projecting spines. The body has a digestive system that 

 is distinct from the interior cavity (cf. 334) ; it has also 

 a simple nervous system, and the beginnings of a blood 

 system. 



FIG. 259. Starfish. 



