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 Canal 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 anal 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 space, 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 mass 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 canal._

a. _Mouth._

b. _Esophagus_, in the fish a very short tube.

c. _Stomach_, white and muscular, beginning with a very short esophagus and ending as a blind sac. If it is much distended, open it to see what the fish may have eaten.

d. _Small intestine_, thin-walled, tubular, and somewhat coiled.

e. _Large intestine_, a short, thin-walled expansion at the posterior end of the small intestine; usually less than half an inch long.

f. _Coeca_, from two to several small pouches attached where the small intestine leaves the stomach.

4. _Spleen_, a reddish brown _globule_ between the folds of the intestine.

5. _Swim bladder_, an elongated chamber lying against the backbone, partitioned off from the cavity below by a delicate membrane.

6. _Peritoneum_, the delicate, silvery membrane which lines the abdominal cavity and enfolds the viscera. Note its spots of pigment.

7. _Pericardial chamber_, the chamber around the heart; see § 1 above.

8. _Heart._ As the fish is placed belly upward in the pan the _ventricle_ faces you, pink, conical, and muscular. Posterior to it, on the dorsal side, is the _auricle_, a membranous sac.

9. _Ventral aorta_, arising on the anterior surface of the ventricle as a white muscular "cord" (really a tube) which is enlarged close to the heart into a bulb, the _arterial bulb_. You should follow up this aorta until you see it divide right and left to send its branches outward into the gills, the branches being called _gill arteries_.

_Questions._

1. The fish frequently swallows its food alive. Why should the stomach be muscular? Why is it better that the intestine does not leave the stomach at the end opposite the esophagus?

2. Of what use can the coeca be? What structure of the human intestine do you recall that is at all like them in form or use?

3. How many times the length of the body is the length of the alimentary canal? Does this indicate that the fish is compelled to eat a great deal of poor food or that its food is highly nutritious, so that little need be taken?

4. Near which end of the fish's body is the heart? Is this the usual or the unusual condition among animals you know about? What advantages can you think of in this arrangement?

5. What advantages are there in having the heart in a chamber separated from the other vital organs?

6. Of how many chambers does the heart consist? Why should at least one of them be muscular?

7. How many times does the blood pass through the heart in making a complete circuit of the body? Would you call this a single or a double circulation?

8. Does the heart force the blood onward or does it draw blood into itself, _i.e._ is the heart a force pump or is it a suction pump?

9. How is circulation made complete? If the heart is a force pump, is its power sufficient to drive blood through artery, capillary, vein, and into auricle, if the capillaries can stand the pressure, or is another action concerned? If it is a suction pump, why does the blood leave the heart?

_Suggested drawings._

a. The body cavity, with viscera undisturbed.

b. The alimentary canal extended.