PISCES 261 



cells and nerves joining either a spinal or cerebral nerve. Phos- 

 phorescent organs are used to see prey, to dazzle and frighten 

 enemies and as recognition lights. 



Harvey ^° has studied luminescence in two species of fish {Photo- 

 blepharon and Anomalops). In these forms the luminescent organs 

 consist of a large number of sets of parallel gland tubes. The 

 lumina of the tubes are filled with an emulsion containing granular 

 and rodshaped bacteria living in sy?nbiotic relation. 



Adaptation of Fishes to Their Environment. — Fishes have their 

 head, body and tail compressed into a curved spindle form which 

 offers little resistance to the water. They have a well-developed 

 caudal fin used for propulsion; paired pectoral and pelvic ^^^zj- are 

 important in executing lateral movements, while their dorsal and 

 ventral fins aid in preserving an even keel. Fishes swim, leap, flop 

 and plane. The climbing perch moves up tree trunks by small 

 hooks on its pectoral fins. The sea robin uses its pectoral fins in 

 progressing over the sandy bottom. The mucus secreted by the 

 glands of the skin and the fact that the scales overlap, tend to reduce 

 friction in swimming. In many forms the swim bladder furnishes a 

 means of varying the specific gravity so that the animal can rise or 

 sink in the water. Some fishes are provided with sharp spines and 

 poison glands for offense and defense. Well-developed organs of 

 vision, olfaction and the lateral line sense enable fishes to locate 

 food or warn them of danger. 



Economic Importance of Fishes and Their Relatives 



Positive. — Cyclostomes were formerly used as food. 



Elasmobranchs. — Dogfish, sharks and skates are eaten fresh and 

 canned. Shark leather, formerly used for " shagreen " spectacle 

 cases, and also for polishing wood and other materials, has recently 

 been advertised widely by the United States Bureau of Fisheries for 

 use in making shoes, handbags and pocketbooks. Attempts have 

 also been made to utilize other elasmobranch skins. Shark fins 

 have been used as gelatin In China and India. Sharks are also used 

 as fertilizer. Shark liver oil has been used as a substitute for cod 

 liver oil. A basking shark may produce over a ton of oil. Dogfish 

 oil is now purified and used as an insect repellant. The odor is not 

 obnoxious to man. 



10 Harvey, E. N. 1920. The Nature of Animal Light. J. B. Lippincott Co., 

 Philadelphia. 



