Echinodermata. 179 



To show the real nature of the plates and their relation to the 

 integument, proceed as follows : 



a. Handle a starfish which has been decalcified, i.e. has had 

 its calcareous matter removed by very weak (two per cent) nitric 

 acid, chromic or other acid. Observe that the body wall is still 

 present, but lacks the hard parts. 



b. Examine a microscopic section of a decalcified ray of a 

 young starfish ; in such section it should be more clearly seen 

 that the calcareous plates are wholly within the integument. 



c. To show still further the relation between the plates and 

 the integument, prepare a thin section of a calcareous plate, as 

 follows : select some pieces of a starfish (left from previous dis- 

 section) ; boil a few of the larger plates 'in caustic potash in 

 order to remove all the organic matter ; wash, and when thor- 

 oughly dry, smooth down one side on a fine file ; polish on a 

 perfectly clean oilstone ; cement the surface of the plate to a 

 glass slide by means of a drop of Canada balsam which has been 

 boiled on the slide, until, on becoming cold, it is with difficulty 

 indented by the thumb nail. Proceed then to plane off, by 

 means of a file, and when quite thin, scrape carefully with a 

 sharp knife, finally smoothing it on an oilstone. The speci- 

 men should be examined from time to time under the micro- 

 scope, in order to ascertain when the proper degree of thinness 

 has been reached, pissolve the balsam by means of turpentine, 

 or better, if properly managed, melt the balsam over a lamp, 

 and carefully push the section into a watch crystal containing 

 turpentine; when thoroughly freed from balsam, carefully brush 

 it with a camel's-hair brush, and mount in Canada balsam in 

 the ordinary manner. 



5. Observe the arrangement of the plates and spines in dif- 

 ferent regions of the body wall. Along the middle of the oral 

 surface of each ray may be seen the shriveled remains of the 

 tube feet, or ambulacra. The region in which they lie is the 

 ambulacral area. The plates along this tract are the ambulacral 

 plates. One row of plates on each side of these ambulacral 



