[CHAPTER XXIV]

BRANCH CHORDATA (Continued): CLASS PISCES (THE FISHES)

THE GOLDEN SUNFISH OR PUMPKIN SEED (Apomotis sp.)

Technical Note.—The species of sunfish named, or some closely related species, can be obtained in any brook or stream in the United States. Gibbosus lives in all streams north of Dubuque, Chicago, Pittsburg, and along the eastern coast north of Charleston. Closely allied species live in all the other parts of the country except in the higher Rocky Mountains west of Bismarck, Pueblo, and Santa Fe. One species is found in the streams of California, but none occurs in Washington or Oregon. In the few places where a sunfish cannot be had, any species of bass or perch may be used. Sunfish live in ponds and sluggish streams in deep holes under a log or at the foot of a stump. They take eagerly a hook baited with a worm, or they may be caught in nets. When sunfish cannot be kept fresh for study in class, specimens may be preserved in alcohol or 4% formalin. But if possible to keep some alive for a time in a jar or tub with plenty of fresh water, the colors of the living fish, together with its manner of swimming and mode of breathing, can be observed.

External structure[16] (fig. [112]).—Examine the general configuration and make-up of the body. Note the deep, laterally flattened trunk and paddle-like tail. The head is closely fitted to the trunk without any neck. Note that the body is thickly covered with firm, hard scales, arranged like the shingles on a roof. Remove one of these scales and examine it under a hand lens. What sort of an edge has it? Such a scale is said to be ctenoid.

The body of the sunfish terminates behind in the caudal fin, a series of cartilaginous rays connected by thin skin and attached to a bony plate at the end of the backbone. Along the median dorsal line will be noted another fin composed anteriorly of spines and posteriorly of soft rays jointed and branched. This is the dorsal fin. How many spines has it? Anterior to the caudal fin on the ventral surface is a median unpaired anal fin. How many spines has it? Anterior to the anal fin are the ventral fins, while on the sides of the body back of the head in a line with the mouth are found the pectoral fins. The ventral fins, attached to a rudimentary pelvis, correspond to the hind legs of the other vertebrates. The pectoral fins, attached to the shoulder girdle, correspond to the arms. In front of the anal fin note a small pit-like opening, the opening from the kidneys and reproductive organs, and just anterior to this a large aperture, the anus. At the anterior end of the head note the broad mouth, surrounded by a complicated system of bones. Note the large eyes surrounded by a series of small bones, the orbital chain. Just anterior to the eyes are two pairs of openings, one pair of each side opening into a closed sac. What are these openings? Note the presence of various bones on the side of the head, each covered with a thin layer of skin. These are membrane bones, characteristic of fishes. Are there any external ears in the fish? Examine the inside of the mouth. Is there a tongue? If so, of what character? Are there teeth? If so, where are they situated?

Note along each side extending to the base of the tail a line of modified scales, on each scale a little mucous tube, the whole series constituting the lateral line. These scales are intimately associated with a large nerve (the vagus), and probably serve an important part, not yet clearly understood, in the life of the fish.

Lift up the flap in front of one of the pectoral fins. This is the opercular flap which covers the gills that lie beneath. Bend this forward and find four gill-arches, each with its double fringe of gills. Note the gill-rakers, short and blunt, on the first gill-arch. Note also on the under side of the flaps turned back, delicate red gill-like structures covered by a membrane. These are the false gills or pseudo-branchiæ, larger in most fishes than in the sunfish. The gills in the fish subserve the same function as the gills of the crayfish, that of purifying the blood by eliminating carbonic-acid gas from it and taking up oxygen from the air mixed with or dissolved in the water. Organs subserving the same purposes in different kinds of animals as, for example, the gills in fish and in crayfish, are called analogous structures. But there is an important morphological difference between the fish's gills and the gills of the crayfish. In the latter animal they are outgrowths of the basal segments of the walking-legs; in the fish they are outgrowths from the alimentary canal. The internal gills of the young toad (tadpole) arise in the same way as those of a fish. Structures which are identical in their origin, like the gills of tadpole and fish, are called homologous structures.