INTRODUCTION. 6 



I 



spicules of silica or other material of which they are 

 composed, especially if these are isolated, a process 

 which is a little troublesome, but is necessary in some 

 eases. The DESMOTHO&ACA or genera with continuous 

 and perforated tests are easily identified. There are 

 various other organisms which may present a Heliozoan 

 appearance, for instance the genus Archerina, Lan- 

 kester, was at first described as belonging to the 

 Heliozoa but is now classed with the Algae ; the 

 genera Nuclearia and Vampyrella, described in the first 

 volume of this work, have been treated by some 

 authors as Heliozoa; also species of Monobia and 

 Ciliophrys may present an Actinophrys-like appear- 

 ance, and the same remark applies to other organisms, 

 but the absence of stiff axes in the pseudopodia or 

 cilia will always (when ascertained) show that such 

 forms are not included in the Heliozoa as defined 

 above. The spores of a few Fungi and some zoo- 

 spores of Algae may also present a superficial resem- 

 blance to the Heliozoa. 



PLASMA. 



This usually consists of a well-defined endoplasm 

 nearly clear and containing only the nucleus or nuclei 



" O / 



and the " central granule ' or centrosome ; this is 

 surrounded by a zone of ectoplasm containing the 

 contractile vesicle, vacuoles, and various inclusions 

 such as food-particles, granules, crystals, and symbiotic 

 zoochlorellaa. 



The occurrence of included crystalline bodies is 

 rare, but food-particles or green zoochlorellae are often 

 present and may be so numerous that the structure 

 of the plasma and the nucleus are entirely concealed. 

 Pulsating vesicles are normally present, but in those 

 individuals and species containing green cells living 

 symbiotically they seem to be as a rule wanting, and 

 if we assume that their function is of a respiratory 

 nature this absence would be accounted for by the 



