CHAPTER V 



THE SUN ANIMALCULE 



ACTINOSPHJERIUM EICHOBNII 



IN many fresh-water 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 animalcule having the structure represented in fig. 29. 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 (a/a-ivos, 

 a ray ; ox^cu/)?/, 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 

 distinguished, but when examined by transmitted light, especi- 

 ally 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 tran- 

 sparent layer, sometimes called the ectosarc, and a central 

 darker medullary portion, sometimes 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 smaller, with much thicker 

 protoplasmic walls, in the endosarc. The protoplasm of the 

 endosarc further shows a difference in that it is more granular 

 and therefore less transparent than that of the ectosarc. 



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