Why does the mussel have no use for head, eyes, or projecting feelers? In what position of the valves of a mussel is the hinge ligament in a stretched condition? How does the shape of the mussel’s gills insure that the water current and the blood current are brought in close contact?
The three main classes of molluscs are: the pelecypoda (hatchet-footed); gastropoda (stomach-footed); and cephalopoda (head-footed). Give an example of each class.
| Comparison of Mollusks | |||
|---|---|---|---|
| Mussel | Snail | Squid | |
| Shell | |||
| Head | |||
| Body | |||
| Foot | |||
| Gills | |||
| Eyes | |||
| Comparative Review.—(To occupy an entire page in notebook.) | |||||
|---|---|---|---|---|---|
| Grasshopper | Spider | Crayfish | Centipede | Mussel | |
| Bilateral or radiate | |||||
| Appendages for locomotion | |||||
| Names of divisions of body | |||||
| Organs and method of breathing | |||||
| Locomotion | |||||
CHAPTER X
FISHES
Suggestions.—The behaviour of a live fish in clear water, preferably in a glass vessel or an aquarium, should be studied. A skeleton may be prepared by placing a fish in the reach of ants. Skeletons of animals placed on ant beds are cleaned very thoroughly. The study of the perch, that follows, will apply to almost any other common fish.
Movements and External Features.—What is the general shape of the body of a fish? How does the dorsal, or upper, region differ in form from the ventral? Is there a narrow part or neck where the head joins the trunk? Where is the body thickest? What is the ratio between the length and the height? (Fig. [209].) Are the right and the left sides alike? Is the symmetry of the fish bilateral or radial?
The body of the fish may be divided into three regions—the head, the trunk, and the tail. The trunk begins with the foremost scales; the tail is said to begin at the vent, or anus. Which regions bear appendages? Is the head movable independently of the trunk, or do they move together? State the advantage or the disadvantage in this. Is the body depressed (flattened vertically) or compressed (flattened laterally)? Do both forms occur among fishes? (See figures on pages [123], [124].)
How is the shape of the body advantageous for movement? Can a fish turn more readily from side to side, or up and down? Why? Is the head wedge-shaped or conical? Are the jaws flattened laterally or vertically? The fish swims in the water, the bird swims in the air. Account for the differences in the shape of their bodies.