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 algae. 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 



