152 PBACTICAIi ZOOLOGY. 



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 dissection). Boil 

 a few of the larger plates in caustic potash in order 

 to remove all the organic matter ; wash, and when 

 thoroughly dry, smooth down one side on a fine 

 file; polish on a perfectly clean oil-stone; cement 

 this 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 oil-stone. The speci- 

 men should be examined from time to time under 

 the microscope, in order to ascertain when the 

 proper degree of thinness has been reached. Dis- 

 solve 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-crys- 

 tal 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 



