36 Marvels of Pond-Life. 



vessel of water, and placed in the light, where, if polyps 

 be present, they will show themselves within twenty- 

 four hours, either attached to the sides of the glass, or 

 hanging from the plants, or suspended head downwards 

 from the upper film of the water. They are elegant 

 objects, and may be kept without difficulty for some 

 weeks. After being confined in a small quantity of 

 water for purposes of examination, they should be care- 

 fully replaced in the larger vessel, and may thus be 

 used again and again without suffering any injury. A 

 low power — a three or two-inch glass — or a one- 

 inch, reduced by employing the erector — is the most 

 convenient for examining the whole creature, but 

 higher powers are necessary to make out its minute 

 structure. They should be viewed with direct and 

 oblique light, as transparent and also as opaque 

 objects. In the latter case the " Lieberkuhn," or 

 polished silver speculum, is convenient, and if the 

 microscope is not furnished with Lister's dark wells, 

 a small piece of black paper may be stuck behind the 

 object, by simply wetting it with the tongue.* 



Although the polyps are remarkable for the sim- 

 plicity of their organization, they do not the less 

 exhibit the wonderful nature of animal life. Their 

 bodies are composed of the substance, called sarcode, 

 in which is imbedded a colouring matter resembling that 

 in the leaves of plants ; every part possesses irritability 

 and contractility, and they are very sensitive to the 

 stimulus of light. The outer layer of their bodies is 



* The side silver reflector is useful for illuminating such objects. 



