THE FISH. 81 



The Gill Covers. 



1. The gill covers are composed of four bones, the 

 opercular bones. 



'i. The upper back one is the opercle. The upper 

 front one is the preopercle. The lower back one is 

 the subopercle. The other bone is the interopercle. 



3. Look below the opercular bones for the bran- 

 chiostegal membrane, which is supported by the bran- 

 chiostegal rays. How many rays are there? Draw 

 the external appearance of the gill covers 



4. See the narrow part of the body between the 

 branchiostegal rays. This is the isthmus. 



The Gills. 



1. Raise one gill cover and find the gills. Find 

 the bone which supports a gill. This is the gill 

 arch. The small threads attached to it are the gill 

 filaments. 



2. The spaces between the gill arches are the gill 

 clefts. How many gill arches? How many gill clefts? 

 How many rows of gill filaments on each gill arch? 



3. Find the gill rakers on the front side of the gill 

 arches. Are all the arches supplied with them? On 

 which'gill are they the longest? 



.4. Find a red line, the gill artery, along the base 

 of the gill filament. 



5. Find a red spot, like a gill, on the inside sur- 

 face of the operculum. It is a false gill, or pseudo- 

 branchia. 



The Scales. 



1. How are the scales arranged? Is there a cover- 

 ing (epidermis) over the scales? Pull a scale out of 

 a yellow or black spot. In what is the color? 



2. Examineoneof the scales after it has been pulled 

 out. Notice radiating striae running from the edge. 



