REVIEW TOPICS ON THE FISH. 89 



The scales on the fish represent the exoskeleton. 

 The bony skeleton inside, to which we refer when we 

 speak of the skeleton, is the endoskeleton. The scales 

 are entirely distinct from this. The scales along the 

 lateral line are perforated, allowing communication 

 with a series of lateral line sense organs, whose nature 

 is not satisfactorily known. Perhaps their function is 

 somewhat similar to that of the ear. 



In the skin are located many muciparous glands 

 which furnish a secretion that lubricates the fish in 

 its passage through the water. This is what makes a 

 freshly caught fish feel so slippery. 



In the catfish, and in many other kinds, the skin is 

 naked and the exoskeleton is undeveloped. In the 

 sturgeon, the exoskeleton is developed as a series of 

 bony plates. 



Review^ Topics on the Fish. 



1. Name all the fins of a fish. 



2. Describe the structure of a fin. 



3. What are the two kinds of fin rays? 



4. How are the scales arranged? 



5. What is the lateral line? 



6. How many scales in the lateral line? 



7. What is the principal organ of locomotion? 



8. Name the opercular bones. 



9. Describe the structure of a gill. 



10. State the direction of the water as it goes over 



the gills. 



11. What makes the water flow over the gills? 



12. What does the fish breathe? 



13. Why does a fish lie with its head up stream? 



14. Describe the course of the blood through the 



body, naming all parts of the heart. 



15. Is the circulation double or single? 



16. Is the circulation complete or incomplete? 



17. What is the false diaphragm? 



