PRINCIPLES OF ANIMAL BIOLOGY 35 



sexual organs in the same individual are known as momrcious or her- 

 maphroditic animals. Make a list of at least ten species of animals which 

 are dioecious. The anatomy of the sexual organs of a dioecious species 

 will be studied later, in the exercise on breeding habits. 



C. Parthenogenesis 



The eggs of certain species of rotifers, crustaceans, insects, and others 

 normally develop without fertilization. 



6. An Aphid. A laboratory experiment will be conducted using the 

 aphid or plant louse, Macrosiphum, and the chrysanthemum as a host 

 plant. Several chrysanthemum plants should be carefully examined to 

 discover if they are free from plant lice. If they are free then a single 

 immature plant louse should be placed on each plant, and the plant 

 should be covered with a lantern globe closed at the top with cheese 

 cloth or muslin. The plants will now be placed on a shelf and cared 

 for by an assistant. Make a record of date and just what was done. 

 After a time interval of a number of days count and record the number of 

 individuals on each plant, the date, the number of days elapsed since the 

 lice were put on the plants. If the interval has been long enough some of 

 the progeny may also have borne young. 



7. Rotifer. At the option of the instructor a second experiment to*be 

 performed by each student may be instituted. Secure from the instruc- 

 tor or an assistant a Syracuse watch glass with a single immature female 

 rotifer in a small quantity of liquid. Examine under a dissector to make 

 sure that only a single rotifer is present. Now fill the dish two-thirds 

 full of distilled water and food materials as" the instructor directs. Write 

 your initials with pencil on the ground edge of the watch glass and put 

 the dish on the shelf designated by the instructor. The assistant will 

 see that dishes are covered and food provided at proper intervals. The 

 record should consist of the date and the number of rotifers put in the 

 culture. After a certain time interval to be announced by the instructor 

 examine your watch glass culture, record the number of rotifers present, 

 date, and interval of time elapsed since the beginning of the experiment. 



How many parents were concerned in the act of reproduction in 

 Macrosiphum? In the rotifer? How can you be sure? Why is this 

 classed under sexual and not under asexual reproduction? If in doubt on 

 the latter point ask for a demonstration of a rotifer showing the egg. 



C. COMBINATIONS OF ASEXUAL AND SEXUAL REPRO- 

 DUCTION WITH DIFFERENCES IN STRUCTURE. 

 ALTERNATION OF GENERATIONS OR 

 METAGENESIS 



Alternation of generations is a phenomenon exhibited in the life cycle 

 of certain animals in which asexual individuals give rise to sexual indi- 



