I POLYSTOMELLA 21 



one side and inwards along the other. While the Polystomella can creep 

 slowly along by means of its pseudopodia the most conspicuous function 

 of these organs is in feeding. When a small food organism comes in 

 contact with a pseudopodium it is paralysed and killed, apparently by 

 the action of some virulent poison secreted by the protoplasm of the 

 Polystomella. Neighbouring pseudopodia move towards the food- 

 particle and by active streaming movements protoplasm accumulates 

 round the food-particle so that the latter is completely enclosed in a mass 

 of protoplasm at it may be quite a considerable distance outside the 

 main body of the Polystomella. Within this the process of digestion is 

 completed and the products of digestion are carried back by the proto- 

 plasmic stream into the body of the Polystomella. 



While any specimen of Polystomella shows the general characteristics 

 mentioned so far, the close scrutiny of a large number of individuals 

 after removing the shell by treatment with dilute acid and staining the 

 protoplasm brings out the fact that the individuals belong to two distinct 

 types — in technical language that the species is dimorphic. The most 

 conspicuous difference between the two is seen in the size of the initial 

 chamber of the shell, i.e. the smallest, first formed, chamber right in the 

 centre of the spiral. In the more frequent type (Fig. 8, i) this (M) is 

 spherical in shape and measures from 60 /u. to 100 /x in diameter. This 

 is known as the megalospheric type. In the other or microspheric type 

 (Fig. 8, 7) the initial chamber and its contained mass of protoplasm (m) is 

 much smaller — only about 10 /t in diameter. Another characteristic differ- 

 ence is seen in the second chamber which in the megalospheric individual 

 is long, curved and somewhat horn-shaped, while in the microspheric it 

 is somewhat oval. Finally the nuclear arrangements are very different. 

 In the megalospheric individual there is a large " principal nucleus " (w) 

 situated just about the middle of the whole mass of protoplasm — i.e. if 

 the spiral were straightened out it would have roughly equal amounts of 

 protoplasm on its two sides. Nuclear material is not confined to the 

 principal nucleus for scattered irregularly through the cytoplasm are 

 small particles and irregular shreds of chromatin. Such particles of 

 chromatin occurring outside the limits of a definite nucleus are termed 

 chromidia. In the microspheric individual there are also chromidia 

 present (Fig. 8, 7, chr), but instead of a single principal nucleus there 

 are numerous small round nuclei which multiply as the t*olystomella 

 increases in size. 



Some of the most striking features of the Polystomella have to' d0 with 

 its life-history, and its dimorphism is found to be associated with two 

 different types of reproductive process which occur in that life-history. 



