Some Microscopic Builders. 



in relation to the shell anatomical details into which 

 we need not enter here, it being sufficient to note that 

 while the Nautilus inhabits but the last-formed chamber 

 of its shell, each chamber of the Foram's shell is formed 

 by, and continues to be occupied by, its own little seg- 

 ment or mass of protoplasm, the number of chambers 

 increasing by a process of budding from the last-formed 

 segment. The wonderful variety of shapes of these 

 minute shells depends upon the plan by which the 

 budding takes place, and two very distinct kinds of 

 shell structure exist, while in one group of Foraminifera 

 the true shell is entirely replaced by a sandy envelope, 

 or " test," the particles of sand of which it is composed 

 being held together by a natural cement exuded by the 

 animal. The two kinds of shell are known respectively 

 as porcellanous and hyaline or vitreous, while the 

 " tests " formed of sand grains are called arenaceous. 

 Under the microscope, when viewed by reflected light, a 

 porcellanous shell bears a strong resemblance to the finest 

 white porcelain ; yet by transmitted light this opacity 

 vanishes, and is replaced by a beautiful brown or* amber 

 colour sometimes tinged with red, while the surface is 

 fretted and sculptured but never completely perforated. 

 The shell of a vitreous or hyaline Foram, on the other 

 hand, has an almost glassy transparency, and every 

 chamber is beset more or less closely with complete 

 perforations passing as small tubular openings direct 

 from the outer to the inner surface. Through these 

 pores the living Foram pushes forth fine strands of 

 protoplasm into the surrounding water, and by their 

 aid food particles are captured and absorbed ; so that 

 every segment of the body within the shell of a vitreous 



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