68 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



c.v. 



ex. 



oes. 



of these the pellicle forms a tough membrane which bounds the surface 

 of the creature and gives it its definite form. In a Paramecium which 

 has been allowed simply to dry up on the microscope slide it may be 

 seen that the surface of the pellicle is not absolutely smooth but is 



sculptured into a minute honey- 

 comb pattern. Beneath the pellicle 

 is a layer of protoplasm which 

 shows a fine striation in a direction 

 perpendicular to the surface, and 

 beneath it in turn is a layer 

 characterized by its containing 

 numerous shining spindle - shaped 

 bodies also arranged with their long 

 axes perpendicular to the surface. 

 These bodies the trichocysts (Fig. 

 28, tr) have a remarkable function. 

 If the surface of the Paramecium be 

 irritated the trichocysts suddenly 

 explode, each losing its spindle 

 form and taking that of a fine 

 needle or filament. The exploding 

 trichocysts, shooting out into the 

 water all round, surround the 

 Paramecium with an impenetrable 

 entanglement which effectually 

 keeps off the attacks of assailant 

 organisms. The fourth or inner- 

 most layer of the ectoplasm con- 

 sists of a spongy protoplasm in 

 the meshes of which water collects 

 from the fluid endoplasm. As the 

 water in this layer increases 

 in quantity it collects especially 

 in two groups of radiating tubu- 

 lar channels situated respectively 

 about half-way between the centre 



and each end of the Paramecium. The water in the radiating channels 

 collects towards their inner ends, and then slowly drains out of the 

 channels into a central drop (Fig. 28, c.v). When this has attained to 

 its full size it suddenly discharges to the exterior, showing itself thereby 

 to be a contractile vacuole. The process of expansion (diastole) and 



n. 



c.v: 



FIG. 28. 



Paramecium. c.v, Contractile vacuole sur- 

 rounded by star of tributary vacuoles ; ex, 

 excretory crystals ; f.v, food vacuoles : m, 

 mouth ; N, macronucleus ; n, micronucleus ; 

 oes, oesophagus ; p, peristome ; tr, trichocysts 

 (some at the upper end of the figure have 

 exploded). 



