MICROSCOPIC PREPARATIONS IN MARINE ZOOLOGY 331 



ing the Odontophore free. The liquid must be poured off, 

 and replaced by several changes of water. The Odontophore 

 can then be laid on a glass slip and straightened out. It 

 will be noticed that it is somewhat folded inwards from the 

 sides, and invested with a very thin transparent membrane. 

 This membrane must be removed with needle and fine 

 forceps. The object can then be mounted, either in balsam 

 or dry. Every species has its characteristic form of teeth, 

 and a skilled zoologist can at once tell to what particular 

 mollusc any one belongs. In some species e.g. Haliotis 

 it is a large and conspicuous object, an inch and a half 

 long by a quarter of an inch wide ; in the common winkle 

 it is a mere thread, and in the great octopus it is not 

 larger than in the whelk. 



There is no need to wait for a visit to the shore to begin 

 work on these. The " whelk " of the street vendor, and 

 the snails and slugs of the garden, furnish beautiful 

 examples. 



Preparing Sections of Shells. There is an interesting 

 study in the structure of shell. The different layers viz. 

 the outer, median, and inner pearly layers have, all their 

 special arrangement in the crystals of carbonate of lime. 



To prepare them, with a small saw cut off as thin a slice 

 of the shell as possible ; then, on a sheet of ground glass, with 

 emery powder and water, grind one surface quite flat 

 begin with medium grain powder, and finish up with the 

 finest " flour emery." Wash the bit of shell well with 

 water. On a bit of thick glass put a drop of Canada balsam 

 (or your balsam and benzole), and hold it over the flame 

 of the spirit lamp until it boils ; then press the smooth 

 surface of the bit of shell well into it, in close contact with 

 the glass. It will be set fast in a couple of minutes. Now, 

 by holding the glass between finger and thumb, grind away 

 the shell on the emery as before, until only the merest 

 film remains. In finishing off with the flour emery wash 



