410 
APPENDIX. 
a circular piece of the shell on one side. If kept submerged in 
a dish of .75 per cent, salt solution, warmed to the temperature 
of the body, the embryo chick may be kept alive for several 
hours to show the beating of the heart, etc. (Figs. 169, 170). 
CHAPTER XXI. 
Protozoa.— As representatives of the Protozoa, Amoeba , Par¬ 
amecium, and Vorticella may be used. They are usually to be 
found in the slimy coating of water-plants— e. g ., pond-lilies, 
etc. They occur in great abundance in aquarium-jars in which 
the water is becoming tainted from the decay of algse. They 
may be cultivated artificially by allowing a dish of marsh grass 
or hay, cut into fine bits and covered with water, to stand in a 
warm place for a few days. To prepare them for observation 
they may be transferred in a drop of water to the glass slide by 
means of a pipette and covered with the cover-slip, with its edge 
resting on a small scrap of tissue-paper or a piece of a hair to 
prevent crushing the specimens. The structure of each organ¬ 
ism should be studied—its body mass of protoplasm, a single 
cell, containing the nucleus, particles of food, and contracting 
vacuoles; the pseudopodia of Amoeba (Fig. 185), and the cilia 
of the other forms; the cuticular covering of Paramecium (Fig. 
188), and Vorticella (Fig. 160), and the muscle-like stalk of the 
latter. Study also their habits; motions of the protoplasm and 
methods of locomotion; feeding; note within the body the 
gradual disintegration of food particles (digestion); look for 
specimens in the process of division (reproduction, Fig. 160); 
notice the sensitiveness of their bodies to contact. If a pro¬ 
longed examination of any specimen be made the animal must 
be kept supplied with water. As rapidly as the water under 
the cover-glass evaporates its loss may be made good by touch¬ 
ing a drop to the edge of the cover^glass. Capillary attraction 
will draw the water between the slide and the cover, and the 
death of the specimen may thus be prevented. Many other 
forms than those mentioned are likely to be found, almost any 
of which will illustrate the essential features of the structure of 
Protozoa. 
Sponges.— Because of the delicacy of their tissues, the study 
