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c. A scale, showing its minute structure.
d. A dorsal or a lateral view of the head, showing the sense organs.
The Mouth and the Gills of the Fish
_Materials._
The same materials as those used in the preceding exercise may be used here.
_Directions._
The mouth, its structure and its action, can be seen by pulling the upper jaw upward and forward until the mouth and the gill chambers open fully. Examine the structure and action of the jaws, the tongue, the throat, and the teeth on each jaw and on the roof of the mouth.
Investigate the breathing apparatus from the throat side and from the exterior, noting the number, form, and structure of the gills, their attachment and their protection.
The mouth may be kept open by a short splinter or a ball of paper.
The pupil should identify the following structures:--
1. _Gill_, an organ for breathing the air dissolved in water.
2. _Gill arch_, an arch of bone or cartilage supporting the gills.
3. _Gill filaments_, fringe-like structures attached to the gill arches, forming the gills.
4. _Gill raker_, lateral projections from the gill arches.
5. _Gill-slits_, openings between the gill arches for the pa.s.sage of water.
6. _Operculum_, the flap-like covering of the gills on each side of the head.
_Questions._
1. Compared with the size of its body, how wide can the fish open its mouth? What do you infer as to the size of its "bite"?
2. Are the jaws rigidly affixed to the skull? Why should they be so attached, or why not?
3. Of how many pieces is the upper jaw composed? the under jaw?
4. Where are the teeth? Judging from their form, size, and situation, what do you think must be their use?
5. Do you think the tongue is used to a.s.sist in mastication? in tasting? in speech? in swallowing?
6. How many gills are there, and where are they situated? How are they attached? Which one is not free from the body throughout its length?
7. What probably causes the color of the gill filaments? What is there in their number and texture which fits them for their function?
8. What is the direction of the water current through the gill chamber? Of what use are the gill rakers?
9. How are the gills protected?
_Summary._
Write a complete account of how the fish eats and how it breathes.
_Suggested drawings._
a. A front view of the fish's face, with the mouth fully open.
b. A side view, as above.
c. A ventral view of the head, with both gill-chambers wide open and the gills separated from each other. Indicate currents by arrows.
d. A single gill.
The Alimentary Ca.n.a.l and the Circulatory System of the Fish
_Materials._
Small fresh fish, shallow pans or dishes of water, forceps, and scissors.
_Directions._
If the instructor has not opened the fish previously, this is to be done by the student as follows: On the ventral side, insert the scissors in the vent (in front of the a.n.a.l fin) and cut straight forward to a point between the opercula. Care must be exercised in opening the chamber about the heart; this lies between the gill chambers.
The various organs, so far as possible, should be carefully drawn out and separated, in order that their structure may be distinguished.
The pupil should identify the following parts:--
1. _Body cavity_, the entire internal s.p.a.ce, divided by a membrane, _false diaphragm_, into a large _abdominal cavity_ and a small chamber, _pericardial chamber_, between the gill chambers.
2. _Liver_, a large red or pink ma.s.s lying at the front end of the abdominal cavity, and divided into two unequal lobes. The _gall-bladder_, thin-walled and green, may be seen between these lobes.
3. _Alimentary ca.n.a.l._
a. _Mouth._
b. _Esophagus_, in the fish a very short tube.
