544 THE MICROSCOPE. 



Helix pomatia has 21,000, and its comparatively dwarfed 

 congener, H. obvoluta, no less than 15,000. When it is 

 remembered that these estimates refer to series of forms, 

 curiously carved and sculptured, the total area sustaining 

 them not measuring in most of the molluscs half an inch in 

 length, we must be filled with admiration at the marvellous 

 creative power bestowed upon the organization of these 

 lowly-creeping creatures. 



The Preparation of Teeth of Mollusca. — The method of 

 preparing the lingual membranes of Mollusca is as fol- 

 lows : — The animal having been taken from its shell, pin 

 down the muscular foot to a piece of cork, pour water upon, 

 it, and let the water be changed as often as it becomes 

 turbid. Then with a dissecting microscope and a good 

 bull's eye condenser cut open and expose to view the floor 

 of the mouth ; pin back the cut edges throughout its 

 whole length, and work out the dental band with knife 

 and forceps. When the band is detached place it in a 

 "watch-glass, and again clean it well by repeated washings 

 and a camel's hair brush : then place it in weak spirit and 

 water, where it must remain for a few days before mount- 

 ing. If the membrane is dense and fatty it must be soaked 

 for a time in liquor potassce, and when removed carefully 

 washed. The best fluids for mounting are glycerine, weak 

 spirit and water, Eimmington's glycerine-jelly, or Goadby's 

 solution. Canada balsam renders them so very pellucid 

 that the finer teeth are completely lost in it. 



Thread-cells. — These curious appendages, so commonly 

 met with in the Actinozoa, and in the tentacles surround- 

 ing the mouth of the Medusas, are also seen in some 

 species of Mollusca. 



These prehensile threads, now generally termed " urti- 

 cating organs," were discovered in 1835, in the Hydra, by 

 Corda and by Ehrenberg. About the same time they were 

 found by R. Wagner in the Actiniae, who at first regarded 

 them as zoosperms. Subsequently, however, he recog- 

 nised their identity with similar organs in the Medusae, 

 and gave them the name of urticating organs. Since then 

 numerous observations have shown that these organs' exist 

 in the entire class of polypes ; in that of the Hydra, 

 Medusas, as well as in the Synaptae, many Turbellarise, 



