410 THE MICROSCOPE. 



" Sertularia filicula forms an interesting object; the 

 waved stem becoming of a dusky red, whilst the cell 

 assuming but little colour, render their mutual relation 

 very obvious. Sertularia abietina is also a fine object, 

 especially when loaded with vesicles, as it often is in the 

 autumn. Plumularia falcata acquires fresh beauty under 

 polarised light, the cells being a pale green, whilst the 

 tubular stem becomes of a crimson hue ; thus presenting 

 a feathered appearance. The most splendid tints are ex- 

 hibited by the calcareous structure of the Polyzoa, and of 

 these the Flustra truncata, when viewed on the selenite 

 stage. The Cellularia avicularia is very brilliant when 

 viewed in the same way ; its cells being covered with 

 plates of carbonate of lime, present a fine display of 

 beautiful tints, especially its birds-head appendages." 



The structure of the spines and other solid parts of the 

 skeleton of Echinodermata can only be displayed by making 

 thin sections, in the way described for cutting bone, at page 

 100. But their peculiar texture requires that certain pre- 

 cautions should be taken, to prevent the section from 

 breaking, whilst being reduced to a desirable thinness, and 

 to prevent the interspaces of the network from being 

 clogged by the particles abraded in the reducing process. 

 In mounting the specimens, liquid balsam must be em- 

 ployed, and only a very gentle heat should be applied ; 

 and if, after it has been mounted, the section should be 

 found too thick, it will be easy to remove the glass- cover 

 and reduce it further, care being taken to harden the 

 balsam, as recommended in preparing bone sections. 



