68 STUDIES OF ANIMAL TYPES 



Study carefully the two dorsal fins. The spines of one are 

 sharp, stiff, and un jointed, while the spines of the other are 

 branched at the ends and are jointed, therefore flexible. 

 Determine which fin has the stiff rays and which the soft 

 ones. The membrane connecting the rays is really double, 

 for it is simply a fold of the skin. 



Observe the tail fin. Is it symmetrical? It is known 

 as a homocercal fin because its lobes are equal and the 

 backbone does not extend into it but stops at its base. 



Note the shape and position of the eyes. Note the ab- 

 sence of eyelids and the transparent membrane, cornea, 

 covering the eyes for protection. Note the dark central 

 pupil and the colored circle surrounding it, the iris. 



Note the nostrils in front of each eye. The posterior 

 nostril is different in shape from the anterior one. Deter- 

 mine, by probing them with a bristle, if they open into the 

 mouth. 



Note that the body is covered with a thin, slimy skin, 

 the epidermis. Underneath this are the scales. Note their 

 arrangement. Find a place on the body where there are 

 no scales. Remove a scale, being sure to observe which is 

 the free and which the attached ends. Place it under 

 the low-power objective and make a drawing of it to show 

 its shape and the difference between the free and attached 

 ends. A scale with the posterior edge toothed is a ctenoid 

 scale. Are the scales of the perch ctenoid? 



Note the line running from tail to gill cover along the side 

 of the perch. This is the lateral line. Examine some of the 

 scales in this line and determine how the line is produced. 



The large flap on the side of the head is the operculum, 

 or gill cover. The opening beneath the gill cover is the 

 gill opening. The operculum is composed of several pieces 



