158 PRACTICAL ZOOLOGY. 



There will now remain a tough white layer in which 

 are imbedded the calcareous plates which constitute 

 the skeleton. 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 devel- 

 oped in and by the membrane. 



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

 contracting, by means of which motion is effected. 

 Note the perforations in the membrane in its thinner 

 portions between the plates where the aboral tenta- 

 cles 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 protrusion of the inner mem- 

 brane through the middle membrane, these tubes be- 

 ing 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 ad- 

 jacent 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 



