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have cycloid scales, which are small and <br />deeply embedded in the skin. Sunfish, <br />bass and perch scales have rows of spines <br />along the exposed posterior margin. <br />These are termed ctenoid scales. Some <br />fish, like the sculpin and catfish in Col- <br />orado, lack scales. <br />Scales provide a very valuable tool in <br />determining the age of fish. Scales form <br />rapidly after a fish hatches, and grow in <br />size as the fish grows. Small rings are laid <br />down along the edges of the scale as they <br />grow. Fish grow faster in the summer than <br />in the winter. Thus, the rings are farther <br />apart in the summer than in the winter. <br />Each annual series of rings represents one <br />year in the life of that fish. These scales <br />can be read to determine the fish's age. <br />Spines of catfish can be read in the same <br />manner. <br />Fins are either single or paired. Single <br />fins are located along the top, bottom and <br />tail of the fish. The dorsal fin may be <br />single, or divided into two parts, and is <br />located on the top of the back. Behind the <br />dorsal fin, trout have a fleshy fin called an <br />adipose fin. The tail fin is termed the <br />caudal fin. The anal fin is located on the <br />belly behind the anus. There are two sets <br />of paired fins, with one fin on each side <br />of the fish. The pectoral fins are located <br />just behind the gill cover and are attached <br />by the "shoulder" bones to the skull. Fish <br />at rest continually use the pectoral fins to <br />backwater which offsets the forward thrust <br />produced by expulsion of water from the <br />gills. The pelvic fins are located on the <br />belly of the fish. The pelvic fins are sup- <br />ported by bones which are suspended in <br />the belly muscles. The pelvic fins are <br />located on the rear of the belly in soft- <br />rayed fish, and under the pectoral fins in <br />spiny-rayed fish. <br />Fish swim principally by muscular <br />movements of the body. The body mus- <br />cle and caudal fin exert the power. Pec- <br />toral and pelvic fins provide steering and <br />maneuvering, while dorsal, anal and even <br />the caudal fin provide stabilization. <br />PREOPERCLE <br />PREMAX <br />NAPE <br /> <br /> <br />MANDIBLE`., <br />LLARY <br />-- -....OPERCLE <br />SUBOPERCLE <br />000, <br />NTEROPERCLE <br />MAXILLARY <br />The organization of the digestive tract <br />varies from group to group. In many fish, <br />the intestine is fairly short and includes <br />only one single s-shaped loop. In other <br />species, the intestine is very long with <br />many coils and loops. In general, fish with <br />very long intestines tend to be mostly her- <br />bivorous in nature. Fish with the shorter <br />intestine usually prefer plankton, in- <br />vertebrates or other fish as foods. The lin- <br />ing of the body cavity (the peritoneum) <br />which holds the viscera, may be dark or <br />silvery, with or without speckles. Fish <br />which consume large amounts of plant <br />material tend to have a very dark <br />peritoneum. Both the size of the intestine, <br />and the color of the peritoneum, are <br />important characteristics used in the iden- <br />tification of minnows and suckers. <br />This description of fish structure has <br />been a very simple overview. Many text- <br />books and papers are available for more <br />information on ichthyology, the study of <br />fish. <br />10 <br />Colorado's Little Fish