CHAPTER V 



THE SUN ANIMALCULE 



ACTINOSPHJERIUM EICHORNII 



IN many freshwater pools, particularly in the warm months, 

 a number of glistening white specks, about as large as the head 

 of a small pin, may be seen floating in the water. Examined 

 under the microscope each of these specks is found to be an 

 animalcule having the structure represented in fig. 31. It has 

 a spherical protoplasmic body, from which radiate out on all 

 sides a number of stiff ray-like processes, the pseudopodia, 

 whence the animal derives its name Actinosphaerium (CIKTH/OS, 

 a ray ; o-faipa, a sphere). Examined more closely, the 

 spherical body is seen to be composed of naked protoplasm, 

 so richly beset with clear spaces or vacuoles that the animal 

 may not inaptly be compared to a speck of foam. It must be 

 borne in mind, however, that the vacuoles of Actinosphaerium 

 differ from foam bubbles in containing not air but fluid. When 

 viewed by reflected light Actinosphaerium appears brilliantly 

 white, because of the reflection of light from all parts of its 

 body, and the details of its organisation cannot well be dis- 

 tinguished, but when examined by transmitted light, especially 

 if it be subjected to gentle pressure, the details of its structure 

 can easily be made out. The body is then seen to consist 

 of a peripheral or cortical lighter and more transparent layer 

 called the ectosarc, and a central darker medullary portion 

 called the endosarc. The difference is really due to the size 

 of the vacuoles, which are large and bounded by very thin 

 envelopes of granular protoplasm in the ectosarc, but are 

 smaller, with much thicker protoplasmic walls, in the endosarc. 

 The protoplasm of the endosarc shows a further difference in 

 that it is more granular, and therefore less transparent, than 

 that of the ectosarc. In any collection of Actinosphaeria there 

 is a considerable difference in size between individuals. 



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