Animal communities, associations of many animals of the same species in communities in which there is a greater or less division of labor. Gregarious, associations where there is but little division of labor. Parasitism, an association where one animal lives at the expense of the other. The animal on which the parasite lives is called the host. If there are two hosts during the life cycle of the parasite, the second host is called an intermediate host. Symbiosis, an association where two animals live together in mutually helpful relations. Commensalism, an association where two animals live together in relations not mutually helpful but without injury to either.

Observations and questions.

Note.—To find answers to many of these questions it will be necessary to refer to the reference books in the laboratory.

  1. Examine a specimen of Volvox. Why may this be considered a colonial protozoan and not a many-celled animal? What is gained by the colonial habit?
  2. Is the colonial habit common or rare in sponges and cœlenterates? What is chiefly gained?
  3. Describe the community life in one of the insects in each of the following groups:—
    1. ant, honeybee, termite.
    2. bumblebee, paper wasp, hornet.
    3. mining bee.
    4. carpenter bee, mud wasp, digger wasp.
  4. Name the host or hosts in the following cases: trichina, liver fluke, malarial parasite, tapeworm, hook worm. Give the life history of one or more of the parasites just enumerated. What is the effect of parasitism on the structure of the parasite?
  5. What is the relation between ants and plant lice? Show how this relation is mutually helpful. Mention other cases of symbiosis that you have come across.
  6. With what animal are barnacles often associated? What is the habit of the pea or oyster crab? What are "guest bees"? What structure is lacking that is found in other bees? What are often found in the cavities of sponges? Why are these associations called commensalism rather than symbiosis?
Summary.
  1. Into what groups can animal associations be divided based upon the number of species concerned?
  2. From the standpoint of protection, is this a good or a bad method of protection?
  3. What disadvantages can you see in this method of protection.

D. Protective Habits and Powers

Materials.

Specimens, charts, and books, showing habits of invertebrates.