PARAMECIUM 191 



length of the animal, and rather to one side so as to lie in 

 the cortical layer or ectoplasm. It is called the Macronucleus, 

 to distinguish it from a smaller ovoid body lying alongside of 

 it and called the Micronucleus. There are two contractile 

 vacuoles, one placed at about the end of the first quarter, the 

 other at the end of the third quarter of the length of the body, 

 in the ectoplasm on the dorsal side. We may now consider 

 these structures in detail. 



Examination with a very high power of the microscope shows 

 that the cuticle is mapped out into a number of very minute 

 hexagonal areas, from the centre of each of which a single cilium 

 projects. These areas are slight elevations of the cuticle 

 separated by fine grooves from one another, and they are so 

 disposed that, under a lower magnification, the grooves appear 

 as two sets of lines crossing each other obliquely and sweeping 

 round towards the peristome and mouth in the manner indicated 

 in fig. 41, C. (The lines are drawn much too large, for clear- 

 ness' sake.) Since each elevated area bears a single cilium, it 

 is obvious that the arrangement of the cilia corresponds exactly 

 with that of the apparent striation, and the figure therefore gives 

 a clear idea of the manner in which the cilia are distributed over 

 the body. It will be observed that in the region of the mouth 

 and peristome they are arranged so as to create a current in 

 the direction of the mouth. 



The cuticle itself is not a fine homogeneous membrane or 

 pellicle like the similarly-named structure in the Flagellata, but 

 appears rather to be of the nature of a specialised external 

 layer of the protoplasm in which, on the alveolar theory of 

 protoplasmic structure, the minute alveoli are arranged side by 

 side, so that their outer walls form a definite surface layer. 



The cortical layer or ectoplasm underlying the cuticle con- 

 sists of a dense semi-fluid protoplasm with a fine alveolar 

 structure. It is relatively of considerable thickness, and contains 

 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. 41, 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, 



