i62 COMPARATIVE ANATOMY 



opposite extremity is blunt and obliquely truncated, and, as it 

 is always directed forward in swimming, it may be called the 

 anterior end. From it projects a single thread-like transparent 

 process of protoplasm in which no definite structure can be 

 made out even with the highest powers of the microscope. 

 This process is the flagellum, and although it presents no 

 obvious structure, it is highly contractile, and when the animal 

 is in active movement executes a series of rapid bending move- 

 ments, which can best be compared to the motion of the 

 finger in beckoning. By these movements of the flagellum 

 the organism is propelled through the water in the direction of 

 the flagellum, and the latter organ has sometimes been called 

 a tractellum, because it draws the organism after it. A little 

 observation shows that the flagellum does not spring from the 

 surface of the anterior extremity, but emerges from a conical 

 depression situated somewhat excentrically on the blunted 

 snout-like end. The flagellum originates from one of the walls 

 of this depression and clearly is a process of the ectoplasm, 

 since it cannot be traced deeper than the most external firmer 

 layer of the protoplasmic body. Though Euglena viridis has 

 the characteristic shape here described, it is eminently con- 

 tractile, and therefore capable of considerable change of shape. 

 An individual, if watched for a short time, may be seen to go ■ 

 through the various phases of contraction shown in fig. 34. 

 Now it is swelled up at the hinder, now at the anterior ex- 

 tremity, and now it is swelled up in the middle so as to 

 resemble a top. These contractions are combined with 

 sinuous worm-like movements, the whole being so character- 

 istic that the name " euglenoid " is applied to them, a word 

 which we have already used in describing the movements of 

 Monocystis. Just as in the last-named genus we saw that the 

 contractile power resided in a specialised external layer, the 

 ectoplasm, so we find that the cell body of Euglena is com- 

 posed of an external layer of firmer and denser ectoplasm, 

 whilst the central more fluid portion of the cell-body is known 

 as the endoplasm. In addition to the ectoplasm, Euglena is 

 clothed by a delicate external cuticle, which is continued at 

 the anterior end into the conical depression from which the 

 flagellum springs. This cuticle exhibits a number of fine 

 oblique parallel striations, which are thought by some authors 

 to be the seat of the contractile movements — to be specially 



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