VORTICELLA 169 



Notice the partial radial symmetry of the animal ; this body- 

 form is due to its sessile habit of life. Paramecium, which is a 

 rapidly moving animal, is not radially symmetrical. Can you 

 explain why a sessile organism tends to be radial? 



Exercise l. Draw a careful outline of the expanded animal on a 

 large scale, and another of the contracted animal, and label 

 the parts above mentioned. 



Study the structure of the body. It consists of a single cell, 

 as does Paramecium, and is composed of two protoplasmic 

 layers, the ectosarc, which is the firm external layer, and the 

 endosarc, the more fluid protoplasm of which the inner portion 

 of the animal is composed. Covering its outer surface is the 

 cuticula, which, with the ectosarc, gives the animal its perma- 

 nent shape. The stalk is a continuation of the ectosarc and of 

 the cuticula. Its inner portion alone, i.e., the axis, is con- 

 tractile; its cuticula simply accommodates itself by assuming 

 a spiral shape. Note the longitudinal striations in the ectosarc 

 at the base of the bell. 



Observe the granular nature of the endosarc and the spher- 

 ical food vacuoles within it ; note the circulation of the latter in 

 the granular protoplasm. Each food vacuole is composed of 

 particles of organic matter in a minute globule of water, which 

 collect in the oral groove and are then driven into the mouth. 

 Watch the formation of them ; this is done easily by placing 

 grains of indigo or carmine in the water. 



Vorticella has a single pulsating vacuole, which is in the upper 

 part of the body. It is the organ of excretion of the animal and 

 consists of a globule of clear liquid which collects near the 

 surface of the body and is then discharged through the ectosarc 

 into the water. As in Paramecium, the water which is ingested 

 as a part of the food vacuoles is discharged through the pulsat- 

 ing vacuole together with renal products. Time the formation 

 of the pulsating vacuoles ; how many form a minute ? 



