A Guide for the Study of Animals Part 32

_Paired fins_, fins occurring in pairs of which the more anterior are the _pectoral_ fins and the posterior are the _pelvic_ fins.

_Fin rays_, the framework or skeleton of the fins over which membrane is stretched to form the fins. Fin rays are of two kinds: those composed of _bone_ and those composed of _cartilage_.

_Lateral line_, a sense organ extending along each side of the fish in a line indicated by tubes or perforations in the scales.

_Gills_, respiratory organs adapted for taking oxygen from the water.

_Operculum_, the flaps covering the gills on each side of the head.

_Pigment_, a substance which gives color to an object.

_Observations._

_Locomotion._

1. Watch the fishes in the large aquarium and determine which fins are most used and how they are used (a) in swimming forward, (b) in swimming upward and downward, (c) in maintaining balance, (d) in remaining at rest, and (e) in guiding the movements of the fish.

2. What advantages are there to the fish (a) in the power to open and close the dorsal and a.n.a.l fins, (b) in having no neck, and (c) in having a compressed form?

3. Enumerate the various ways by which the body of the fish is adapted to rapid movement through the water.

_Feeding._

4. What is the food of the fishes you are studying? Feed them and watch them eat. Why is the upper jaw often called a "lip"? What is the shape and size of the mouth when opened in feeding? Does the fish chew its food? Describe in detail the fishes' method of feeding.

_Respiration._

5. Identify the opercula and the gill openings. Watch the movements of the opercula and mouth, and determine what movements are concerned in breathing and their order. Describe in detail the circulation of water used in breathing and how it is caused.

_Sense Organs._

6. Identify the eyes, nostrils, and lateral line. How many nostrils are there and where located? What is the position and extent of the lateral line?

7. Describe the location of the eyes. What is the shape of the outer surface of the eyes? Why this shape? Can the eyes be moved, _i.e._ can they be rotated, rolled, or retracted? From what direction might an enemy approach without being seen? How would such an enemy be detected?

_Protection._

8. With what protective structures is the body covered? Do they hinder the movements of the fish? What are the advantages of the scale covering of fishes over the sh.e.l.l covering of gra.s.shoppers or crayfishes?

9. In what other ways are the fishes you are studying protected against enemies? Since you cannot account for the red color of goldfishes on the basis of use to the fish, then how do you account for this bright color?

_The Body._

10. What is the symmetry of the fish? Into what regions is the body divided?

_Summary of the study of the living fish._

Enumerate in one column the different adaptations which fit the fish for life in water and in a second column state the special purpose of each adaption.

The External Structure of the Fish

_Materials._

Freshly killed or preserved fish in dishes or shallow pans with enough water to prevent drying. Simple or compound microscopes, forceps, and a bristle.

_Directions._

Examine the fins and identify the membrane and the supporting rods, or _rays_, of bone or cartilage. Notice how the ends of the cartilaginous rays keep the membrane from tearing.

Investigate the scales as to their arrangement, number, and size.

Remove a small patch of scales along the lateral line to find how they are attached, where the fish's color is situated, and how access to the sensory organs of the line is permitted. Examine a scale under the microscope.

Observe the eyes and identify the parts similar to those of the human eye: lid, lash, tear-duct, cornea, iris, and pupil.

In front of and between the eyes, find the nostrils. By means of a bristle determine whether these are connected and whether they do or do not open into the mouth or the throat.

_Questions._

1. Make a list of the fins, cla.s.sifying them according to their structure.

2. Bearing in mind the differences in structure and consequent action,--what can you say regarding the adaptation of the several fins for protection? for rigidity or flexibility in locomotion?

3. State how much of the body is covered with scales, and where the largest and the smallest ones are found.

4. How are the scales arranged with reference to each other? What benefit is derived from this in protection? in locomotion? If you have noticed any mucus or slime upon the body, state its use.

5. Do the scales or the skin bear the pigment? Give the color pattern of the kind of fish used in cla.s.s. How would this be useful to the fish in its natural home?

6. Describe the structure of a scale and state how it is attached to the skin. In what way is the lateral-line scale specialized?

7. State how, when the fish is swimming, the nostrils catch odors. By means of a diagram, with arrows show the probable direction of the water current through the nose.

8. State which of the structures of your eye are present in the fish's eye, and which are missing. Could a fish weep? wink? How would a fish sleep?

9. Inasmuch as light penetrates water but a little way, so that objects can be distinguished only within about thirty feet, would the fish be nearsighted or farsighted?

_Suggested drawings._

a. A side view of the entire fish, fully labeled.

b. A bony rayed and a cartilaginous rayed fin.