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 

 follows : Under a dissecting microscope and a large 

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

 floor of the mouth ; pin back the cut edges throughout 

 its length, and work out the dental band with knife and 

 forceps. The band being detached place it in a watch- 

 glass, and boil over a spirit lamp in caustic potash solu- 

 tion. Having by this process freed the tongue from 

 its integuments, remove it, wash it well, and place it for 

 a short time in a dilute acid solution, acetic or hydro- 

 chloric. Wash it well in water, float it upon a slide ; 

 and with a fine sable brush lay it open flat, and remove 

 whatever dirt or fibre may adhere to it. Lastly, place 

 it in weak spirit and water, and there let it remain for 

 a few days before mounting. It is better to mount 

 specimens in glycerine, Kimmington's glycerine-jelly, 

 or Groadby's solution. Canada balsam renders them so 

 very pellucid that the finer teeth are completely lost. 



Thread-cells. These curious appendages, so commonly 

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

 ing the mouth of the Medusae, 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 Actinia, who at first regarded 

 them as zoosperms. Subsequently, however, he recog- 

 nised their identity with similar organs in the Medusa?, 

 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, 

 Medusae, as well as in the Synaptce, many Turbellariae, 



