INVERTEBRATA. 407 



2. Organ of Locomotion. At the blunt anterior end of 

 the body there is a single long delicate protoplasmic pro- 

 cess such as is usually called a flagellum. This whip-like 

 filament is the organ of locomotion, and to its lashing 

 movements the motion of the organism through the water 

 is due. As the flagellum is anterior in the movement of 

 the Euglena, and does not propel the body from behind 

 but draws it from the anterior end, it has been distin- 

 guished by the term tractellum. The tractellum arises 

 from the side of a little conical depression of the surface 

 of the body, which is sometimes called the mouth, and 

 which somewhat resembles the cytostome of Paramecium. 

 In the species of Euglena, however, observers are agreed 

 that no solid particles are ever taken in as food through 

 the mouth. The mouth is continued into the protoplasm 

 of the cell for a short distance as a minute tube, which is 

 for convenience termed the gullet. 



3. Organ of Excretion. Near the inner extremity of 

 the gullet is a cavity of spherical shape, and this was 

 formerly supposed to correspond to the contractile 

 vacuole of other Protozoa. It is not, however, contractile, 

 but permanent and almost constant in size and shape. 

 Careful observation shows that near the surface of this 

 cavity are smaller vacuoles which from time to time 

 contract and disappear. The large permanent cavity, 

 therefore, seems to serve as a reservoir into which the 

 contractile vacuoles open. There is reason to believe that 

 the reservoir communicates with the lumen of the gullet, 

 and that in this way the excreted liquid is conveyed to 

 the exterior. According to Dangeard the true contractile 

 vacuoles increase in size and then fuse together to form a 

 single vacuole which enlarges until it comes into contact 

 with the reservoir, into which it then bursts, and the 

 contents of the reservoir escape gradually through the 

 efferent canal which leads into the gullet. 



4. Stigma and Haematochrome. In contact with 

 what may be called the dorsal and anterior side of the 

 reservoir is a pigment spot known as the stigma, which 



