_Directions._
Select a large worm and cut carefully through the body wall along one side, midway between the dorsal and ventral surfaces, from the anterior end to the posterior. Lay the worm on any convenient fairly soft surface (a piece of pine, cork, peat, paraffin), preferably under water, and pin out the walls so that you can see into the interior.
Identify the structures defined above, as well as the dorsal and ventral blood vessels and the "hearts."
The nephridia are not easily distinguished, though they are very numerous. They are long, slender, coiled tubes, two in each somite, lying in the body cavity, one on each side of the alimentary ca.n.a.l.
If possible, identify them.
Notice that most of the internal organs are free from the body wall, lying free in the body cavity.
_Questions._
1. What is the extent of the body cavity, anteriorly and posteriorly?
What is its shape?
2. What, in general, is the shape of the food ca.n.a.l? How many external openings has it?
3. Into what regions is the food ca.n.a.l differentiated? Suggest one advantage of having these specialized regions.
4. How is the alimentary ca.n.a.l of the worm kept away from the body walls? Why have it thus supported?
5. What is a septum? How many septa are there? What vessels and tubes pa.s.s through a septum?
6. Locate the nerve cord. How long is it? How frequently do the ganglia occur on it? Which end of the living worm is the more sensitive. Suggest the connection between this fact and the location of ganglia.
_Suggested drawings._
a. Earthworm, showing structures mentioned in this study.
Details of Structure--Microscopic Anatomy
_Materials._
Sections of earthworms, preferably both cross sections and dorso-ventral, longitudinal ones.
_Directions._
In a section under a simple lens, identify the dorsal and ventral surfaces, the body wall, the body cavity, the alimentary ca.n.a.l, and, if possible, the dorsal and ventral blood vessels and the ventral nerve cord.
Under a microscope identify the same structures. Notice that the body wall consists of three layers of cells: an outer single layer, the _epidermis_; a middle layer, the _circular muscles_; and an inner one, the _longitudinal muscles_.
The nephridia show as loosely scattered fragments in the body cavity, at the right and left of the alimentary ca.n.a.l.
If you happen to have a section which shows one or more setae, identify the muscles which operate it, and the group of glandular cells at its inner end, which are known as _setigerous_ (from _seta_) cells.
_Questions._
1. Describe the epidermal cells. What is their probable function?
Among them notice larger cells, clear and rounded. These are the _mucous_ (slime) cells.
What is the use of mucus to the worm?
2. Describe the muscle cells. In which direction do the muscle fibers extend? What is their function? Which layer of muscle cells is thicker, the circular or the longitudinal? Why should it be?
3. Notice the cells in the walls of the alimentary ca.n.a.l. What layers do you find? How are they arranged?
4. If the section you are studying is a cross section from the region back of the gizzard, the alimentary ca.n.a.l will look horseshoe shaped, indented from the dorsal surface. What is the effect of this indentation upon the amount of surface in the alimentary ca.n.a.l?
5. Study the cells of the nerve cord. How do they compare in size and shape with the muscle cells?
_Suggested drawings._
a. A diagram of a cross section, showing the relation of the organs.
b. A diagram of a longitudinal section, at least through the body wall, to show the arrangement of muscle fibers.
c. A drawing of a portion of the body wall, to show details.
Summary of Important Points in Study of the Earthworm
1. Compared with a hydra, how many cells has an earthworm?
2. Compared with a hydra, how much are the cells of an earthworm differentiated?
3. How are these differentiated cells usually arranged with respect to one another? What advantage is there in this arrangement?
4. Recall the kinds of work done by paramecium, sponge, hydra, and worm, and at the same time consider also the efficiency of each. Can earthworms do any more kinds of work than any of the others? Can they do any more work? Can they do any of it better? Give the probable reasons for this?
Comparative Study of Worms
_Materials._
As many different kinds of worms as you can get, living or dead.
_Directions._