A FEW FACTS ABOUT ZOOLOGY. 



around 'the margin are tentacles furnished with stinging 

 threads. 



The star-fish is another and higher specimen of the radiates. 

 The star-fishes consist of animals with a flat, central disk, 

 with five or more arms radiating from it. Fig. 2 is a picture 

 of the brittle-star; its long, slender arms are not prolongations 

 of the body, as are many of our star-fishes. 



Coral Polyps. 



These belong to the radiates; they generally grow together 

 in great clusters or colonies. A single polyp, cast off from a 

 group, soon shows the typical form ; that is, the tentacles 

 begin to form around the circular opening, which is both 

 mouth and stomach. There is but the one opening, so that 

 fluids must pass in and out the same way. These tentacles or 

 feelers keep up a constant motion, thus causing the water to 

 flow in and out; the limy portion of the sea water is left 

 behind in the thin plates which radiate from the center of 

 the polyp; a new polyp soon appears in the form of a bud 

 from the first; this develops and adds to the limy formation 

 from the sea water; thus each polyp adds its portion to the 

 bony skeleton ; at the extremity the growth goes on, forming 

 new polyps, while the stem or limy skeleton is left behind, 

 dead. " In radiates," says Agassiz, " we find no prominent 

 bilateral symmetry, but an all-sided symmetry, in which 

 there is no right and left, no front nor back. Radiates are 

 spheroidal bodies; yet, though many of them remind us of 

 a sphere, they are by no means to be compared to a mathe- 

 matical sphere, but rather to an organic sphere, so loaded 

 with life, as it were, as to produce an infinite variety of 

 radiate symmetry. The mathematical sphere has a center 

 to which every point of the surface bears identical rela- 

 tions; such spheres do not occur in the animal kingdom." 

 Still ascending, we come to the next division, the sub- 

 kingdom mollusca. 



