190 COMPARATIVE ANATOMY 



can hardly be the case, since the cuticle can easily be isolated 

 by crushing the organism, and then the striae can be seen 

 to be thickenings of the cuticle and not differentiations of 

 the ectoplasm. The cuticle is obviously very elastic, or the 

 euglenoid movements would be impossible. 



The nucleus of Euglena is a spherical vesicular body, con- 

 taining a darker spot called, but probably incorrectly, the 

 nucleolus. The nucleus maintains a constant position about 

 one-third of the whole length of the animal from its hinder 

 end. Its structure will be described in detail when we come 

 to consider the reproductive process. 



One of the most striking characteristics of Euglena is its 

 green colour. Viewed with a low power of the microscope, 

 the cell-body appears uniformly green, with the exception 

 of the blunt anterior extremity and the flagellum, which are 

 colourless. But examination with a higher power shows that 

 the green tint is due to the presence of a number of circular 

 or oval discs, known as the chromatophors. In many species 

 of Euglena the chromatophors lie in an even layer imme- 

 diately under the external surface, and, since they do not 

 change their position, it is probable that they are embedded 

 in the ectoplasm. In Euglena viridis, however, the chromato- 

 phors radiate outwards from a heap of granules, situated in the 

 centre of the body. The green colour of the chromatophors 

 is due to the presence of chlorophyll, each chromatophor con- 

 sisting of a matrix of protoplasm of firm consistency, through 

 which the chlorophyll is evenly diffused. Just as green plants 

 are able, through the agency of their chlorophyll corpuscles, to 

 decompose the carbonic acid in the air, setting free oxygen 

 and combining the carbon with water to form starch, so 

 Euglena is able through the agency of its chromatophors to 

 decompose carbonic acid, but the product is not starch but 

 an allied body of the same chemical composition viz. C 6 H IO 

 O s . This substance, called paramylum, differs from starch 

 in several respects, particularly in giving no colour with iodine, 

 whereas starch gives a deep blue colour. 



When Euglense are kept in a vessel in bright sunlight, 

 bubbles are abundantly formed in the water in which they are 

 contained, and these bubbles, if collected, can be shown to 

 consist of oxygen. At the same time the heap of granules 

 from which the chromatophors radiate can be seen to have 



