VORTICELLA 189 
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 1. Draw a careful outline of the expanded animal ona 
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 
entosarc, 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 ectosare and of 
the cuticula. Its inner portion alone, 7.e., the axis, is con- 
tractile; its cuticula simply accommodates itself by assuming 
a spiral shape. Note the longitudinal striations in the ectosare 
at the base of the bell. 
Observe the granular nature of the entosare 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 ectosare 
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? 
