THALLOPHYTA: ALGAE 15 



is a short narrow tube, called the gullet, that leads to a spherical cavity, 

 the reservoir. The flagelliim is inserted inside the reservoir and projects 

 through the gullet. Near the reservoir is a contractile vacuole (more than 

 one in some species), which alternately contracts and expands. The 

 contractile vacuole discharges its contents into the reservoir. Con- 

 tractile vacuoles, usually regarded as organs of excretion, are found in 

 many one-celled animals, as well as in the motile cells of many algae. 

 Flagellates show an advance over the blue-green algae in having 

 a definite nucleus. Moreover, their photosynthetic pigments, where 

 present, are always confined to definite plastids. Euglena has many small 

 green plastids (chloroplasts) , but some flagellates, not belonging to the 

 Euglenophyceae, have yellow or brown plastids. All these colored forms 

 carry on photosynthesis. Other flagellates, some belonging to the 

 Euglenophyceae, are colorless and live either as saprophytes, absorbing 

 organic matter in solution through the plasma membrane, or as animals, 

 ingesting solid particles of food either through the gullet or by means of 

 pseudopodia. A few flagellates are parasitic on animals, one of these, 

 Trypanosoma, causing a disease of man known as African sleeping 

 sickness. Some forms with chlorophyll ingest solid food particles through 

 the gullet. Some species of Euglena carry on photosynthesis in the 

 light, but, if kept in darkness and supplied with organic matter in solution, 

 become colorless and saprophytic. In all the Euglenophyceae food is 

 stored as paramylon, a starch-like carbohydrate, and often as oil. The 

 presence of paramylon granules is very characteristic, even in colorless 

 cells. True starch is not formed. Flagellates belonging to other groups 

 of algae differ with respect to the type of food stored. 



Reproduction. As in all flagellates, reproduction in Euglena occurs 

 by fission, the cell dividing longitudinally. In the presence of unfavor- 

 able conditions, encystment often occurs. The protoplasm retracts the 

 flagellum, rounds up, secretes a thick gelatinous covering about itself, 

 and goes into a resting stage. Although later the cyst usually produces 

 a single motile protoplast, sometimes it divides internally into a number 

 of smaller protoplasts that escape, develop flagella, and grow to mature 

 size. Sexual reproduction in the Euglenophyceae is of doubtful 

 occurrence. 



Colacium, a member of the Euglenophyceae, is interesting in being an 

 attached form lacking flagella in the vegetative condition. Its cells are 

 surrounded by a gelatinous wall and are united into small irregular 

 colonies. When reproduction occurs, the cell contents escape as a 

 naked euglenoid protoplast with a flagellum. 



Relationships. Flagellates are related on the one hand to various 

 groups of algae and on the other hand to the Protozoa. The fact that 

 they are intermediate between plants and animals strongly suggests 



