ASTERIAS VULGARIS. 41 



d) Scrape a few of the tube feet from a small space in one of the 

 grooves and observe the arrangement of the ambulacral plates 

 which form its walls, and of the ambulacral pores through which 

 the tube feet had projected. Are the pores in the plates or between 

 the plates ? 



Exercise 3. Draw the oral surface on the scale suggested in Exercise i. 



Technical Note. Study, with a lens, the cross-section of the skeleton of 

 an arm cut with a fine-toothed saw; also a small area of one side 

 of the ambulacral groove from which the tube feet have been removed. 



Note, in the cross-section, the varying thickness of different portions 

 of the firm dermal wall. While the dorsal region is strongly arched, 

 the ventral part is in the form of an inverted V. The arms of the V, 

 ambulacral plates, meet above like the rafters on a roof and form the 

 ambulacral ridge, the apex of which is truncate or slightly channeled. 



The shriveled up inner end of each tube foot can now be seen. This 

 inner end, called the ampulla, was bulb-like in the living animal. 

 Where the tube foot passes through the dermal wall it is connected by a 

 short horizontal branch with the radial water tube which runs the 

 full length of the arm. This water tube can be seen in cross-section 

 as a tiny hole in the ambulacral ridge just above the radial nerve, 

 which, as has been said, appears as a faint line in the bottom of the 

 ambulacral furrow. 



Exercise 4. Draw the cross-section, on a scale of one and one-half, 

 and a group of the ambulacral pores and plates on a still larger scale. 



Technical Note. With a pair of fine-pointed scissors remove the aboral 

 wall, with the exception of the madreporite, from a preserved specimen, 

 taking care not to injure any of the internal organs. In this dissection 

 make an incision from the end of the arm down each side to where 

 it joins the disc, clip out the angle at the junction of the two arms, 

 cut around the madreporite, and then carefully remove the "roof" 



