220 COMPARATIVE ANATOMY 



the more important structures of the cell-body. The two con- 

 tractile vacuoles lie in the deeper part of the cortical layer and 

 have the form and position shown in fig. 47, A. Each contracts 

 with great regularity at intervals of about 10-20 seconds, the 

 contractions of the two not being synchronous but rather 

 alternate. Just before contraction (systole) the vacuole appears 

 as a large clear space in the ectoplasm ; in systole its walls 

 come together and its fluid contents are expelled to the exterior, 

 but no visible aperture for their passage can be detected. The 

 ensuing pause, during which the vacuole is refilled with fluid, 

 is known as the diastole. Immediately after the systole a 

 number, varying from 6-10, of fine canals make their appear- 

 ance in the ectoplasm, radiating from the spot where the vacuole 

 disappeared by closure as the spokes radiate from the hub of a 

 wheel. These canals may reach for some considerable distance 

 into the surrounding cell-body, but those of one vacuole do not 

 communicate with those of the other. The canals become 

 swollen with fluid at their inner ends and slowly void their 

 contents into the vacuole, which reappears, gradually filling with 

 the fluid poured into it by the different canals till it reaches 

 its largest dimensions, and then it suddenly contracts again. 

 Towards the close of the diastole the different canals, having 

 emptied their contents into the vacuole, become altogether or 

 nearly indistinguishable. 



Immediately beneath the cuticle is a layer of spindle-shaped 

 rods lying in the most external layer of the ectoplasm. These 

 rods are known as the Trichocysts. They form a tolerably 

 uniform layer immediately below the whole surface of the cell- 

 body, and are disposed with their long axes at right angles to 

 the surface. Each trichocyst is about 4 /* (p = y^ 1 ^ millimetre) 

 in length, is somewhat darker than the surrounding protoplasm, 

 and betrays no sign of structure, appearing as a simple spindle- 

 shaped homogeneous body, even under the highest powers of 

 the microscope. But under certain circumstances they undergo 

 a remarkable change. If a trace of acetic or osmic acid is added 

 to the water in which a Paramecium is contained the trichocysts 

 are suddenly projected to about eight times their former length 

 in the form of long thread-like filaments. In the genus 

 Paramecium the trichocysts are promptly shot out on stimulation 

 by an irritating fluid, but those of some allied Infusoria 

 can only be made to extend with great difficulty. As a rule the 



