184 Practical Zoology. 



Bend this membrane to see the relations of the calcareous 

 plates to the membrane and to each other. 



20. By picking with the forceps prove that the membrane is 

 continuous over both the inner and outer surfaces of the plates, 

 as well as between them. This is an important point, as the 

 calcareous plates are developed in and by the membrane. 



Part of the membrane, if not all, has the power of contract- 

 ing, by means of which motion is effected. Note the perfora- 

 tions in the membrane in its thinner portions between the plates 

 where the aboral tentacles passed out. 



21. Reviewing what was noticed in the examination of the 

 inner and outer membranes, it will be evident that the aboral 

 tentacles are tubular extensions of the body formed by the pro- 

 trusion of the inner membrane through the middle membrane, these 

 tubes being covered by the outer membrane. 



22. Turn now to the tube feet and their ampullae and make 

 out their relations to each other and to the adjacent parts of the 

 skeleton. The calcareous plates which form the sides of the 

 ambulacral groove are the ambulacral plates. 



23. Pick away a few of the ampullae and then the cor- 

 responding tube feet, comparing the arrangement of the two. 

 In this way clean the ambulacral plates and examine them 

 carefully. 



24. Alternately press the ambulacral plates of the two sides 

 together and separate them to see the range of motion allowed 

 by the joint. Observe the muscles connecting the ambulacral 

 plates of the opposite sides, just inside of the nerve. 



25. In the angle formed by the ambulacral plates, find the 

 cut-off end of the water tube of the ray. Insert in the end of 

 this the point of a drawn-out glass tube, and inflate. When the 

 ampullae are distended, press on them with the finger and note 

 the effect on the tube feet ; with a lens examine the distended 

 ampullae. In fresh specimens the ampullae may be injected 

 with a colored liquid or with gelatine to be kept as permanent 

 preparations. In such preparations and in a microscopic section 



