212 ELEMENTARY ZOOLOGY. 



feet can be attached to the solid surface, and by further bending and 

 further attachment of feet and the combined muscular pull in the 

 fibres of the feet themselves, it can finally right itself. 



The sea-urchins use their spines in moving, since in this case 

 the spines are fitted with a kind of ball and socket joint and have 

 muscles. The brittle stars move about in a slow and clumsy fashion, 

 by using their slender arms as feet. The sea-cucumbers use the 

 ambulacral feet in motion ; or in certain burrowing types they burrow 

 by the muscular movements of the body alone. The sea-lilies are 

 typically attached, like plants, and only have the motion of waving 

 arms or bending stalks. " 



C 



FIG. 66. Diagram showing the arrangement of the nervous matter in the starfish. 

 c, central nervous ring about the mouth (<?) ; r.n, radial nerve in each arm (see r.n. in 

 Fig. 64). 



Questions on the Figure. Compare this arrangement of nervous 

 matter with that seen in Dero (Figs. 56, 57). What similarity? 

 What differences? 



In some cases the tube-feet are modified into tentacles, and it is 

 probable that in most forms they are more or less sensitive. They 

 likely aid the animals in respiration also. Other thin projections 

 of the body wall (Fig. 63, d. b), known as branchiae, aid in respiration. 



231. Reproduction and Development. The sexes are distinct and 

 the eggs are laid in the water, where they are fertilized. They develop 

 into free- swimming gastrulas of very regular form. Later they 

 develop into very characteristic bilaterally symmetrical larvae (Fig. 

 67), which swim freely in the water for a few weeks and then settle 

 to the bottom. They become attached to something and undergo 

 a metamorphosis in which the adult radially symmetrical animal 

 is formed. Radial symmetry is a form of symmetry suiting animals 



