Echinodermata. 185 



of a properly prepared ray, it may be seen that the water tube of 

 the ray sends off side branches to the tube feet, and also that the 

 cavities of the tube feet and ampullae are continuous. By the 

 contraction of the ampullae the tube feet are extended, and by 

 the muscles in their walls they are moved from side to side and 

 applied to the surfaces on which the starfish rests. The end is 

 fixed by means of the suckerlike disk at the tip of the foot to 

 some foreign object ; then, by the contraction of the tube feet, 

 the starfish pulls its body along. 



The water finds its way through the madreporic body into the 

 stone canal, thence to the water ring around the mouth, and from 

 this to the radial canals. The water thus taken in probably 

 serves for respiration as well as for locomotion. 



26. Make a drawing of a cross section of a ray, showing as 

 many as possible of the above-noted points of structure. A slide 

 with a series of very small starfishes shows well how the rays are 

 formed as outgrowths of the disk. 



Read Seaside Studies in Natural History, Agassiz. 



Topics for Reports. Starfishes and the Oyster Industry. 



THE SEA URCHIN. 

 STUDY OF A LIVE SEA URCHIN. 



At low tide search the tide pools for sea urchins. For collec- 

 tion and study, follow the directions given for the study of the 

 live starfish. Keep sea urchins in a salt-water aquarium and 

 study their habits. Turn a sea urchin upside down in the aqua- 

 rium. Can it turn back? How does it accomplish this, and how 

 long does it take to right itself ? 



The requisites for this work are, cleaned skeletons, or tests, 

 alcoholic specimens, microscopic sections, etc., as in the case of 

 the starfish. 



THE CLEANED TEST. 



i. Observe the radial distribution of the parts around an axis, 

 at one pole of which, the oral pole, is a large opening. At the 



