INFUSORIA. 



25 



absorb the air from their viscicles, and thus, becoming spe- 

 cifically heavier than the water, immediately disappear by 

 sinking into the depths of the ocean. By what process 

 they effect the absorption and reproduction of the air in 

 their bubbles, yet remains to be discovered." 



52. Echinus. The shell or skeleton of this animal is 

 well known under the name of 



sea-urchin, or sea-egg, and is 

 represented by Fig. 17. Its 

 form is spheroidal, resembling 

 that of an orange. On the out- 

 side there are a great number 

 of tubercles arranged in double 

 lines in beautiful symmetry, 

 from the mouth downward, and 

 forming meridian lines from one pole of the sphere to the 

 other. All of these are little balls, smooth, and polish- 

 ed on the outside, and which serve for the articulation 

 of the basis of the spine, with which, when alive, this 

 animal is covered. When examined by a magnifier, it 

 will be seen that the end of the spine- has a socket exactly 

 fitting this ball, thus forming the ball and socket joint, 

 which has a universal motion. The head of the spine is 

 furnished with a capsular ligament to keep it in place, and 

 around which are sets of radiating muscular fibres, by 

 which motion in all directions is given it. 



53. The shell is constructed of calcareous matter, and is 

 composed of oblong six-sided plates, accurately fitting 

 each other, and arranged in rows like a mosaic pave- 

 ment, as seen by Fig. 18. There is 



ion by which the globular shell of 

 the echinus is enlarged according 

 to the wants of the internal ani- 

 mal, for some species grow from 

 the size of a pin's head, to six or 

 eight inches in diameter. This is 

 accomplished by dividing the shell 

 into a great number of six-sided 



wonderful provis- 

 Fig. 18. Fig. 19. 



How are the spines of the echinus fitted to its shell? What motion 

 have these spines ? In what manner is the house of the echinus enlarged ? 



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