INFUSORIA. 



absorb the air from their viscicles, and thus, becoming 

 specifically heavier than the water, immediately disap- 

 pear 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." 



Echinus. The shell, or skeleton of this animal is well 

 Fig. 17. known under the name of sea 



urchin, or sea egg, and is re- 

 presented by fig. 17. Its form 

 is spheroidal, resembling that of 

 an orange. On the outside there 

 are a great number of tubercles 

 arranged in double lines in 

 beautiful symmetry, from the 

 mouth downwards, and form- 

 ing meridian lines from one pole of the sphere to the 

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

 ed on the outside, and which serve for the articulation 

 of the bases 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 ex- 

 actly 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. 



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 a wonderful pro- 

 Fig. 18. Fig. 19. vision by which the globular shell 

 of the Echinus is enlarged accord- 

 ing to the wants of the internal ani- 

 Imal, for some species grow from 

 I the size of a pins' head, to six or 

 eight inches in diameter. This is 

 accomplished by dividing the shell 

 into a great number of six sided 



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

 have these spines 1 In what manner is the house of the echinus enlarged 1 

 3* 



