114 LABORATORY STUDIES IN ZOOLOGY 



Hold the fish again on a level with the eyes as before mentioned. 

 What is the relation of dorsal and pectoral fins with regard to 

 their surfaces which are presented to the water? With a similar 

 object in view, observe the eyes, mouth, and other parts. 



If time permits and living specimens of any species are avail- 

 able, observe the movements of the fish in order to determine the 

 important organ of locomotion. Describe the method. By 

 means of rubber bands slipped over the body, bind the pectoral 

 fins close to the sides. Describe results. Perform the same 

 experiment with the pelvic fins. Bind both pairs close to the 

 body at the same time. Results? 



Look on the midventral line just cephalad of the anal fin for the 

 anus or opening of the intestine to the exterior. Behind this is a 

 depression in which the urogenital papilla is located. This 

 papilla bears the small openings of the urinary and genital 

 organs. 



Study more carefully the scaly armor of the fish. Note the 

 relation of one scale to another and the difference in their size in 

 different parts of the body. Scales are formed by and set into 

 pockets of the dermis of the skin, the epidermis being reduced to a 

 thin film covering the free part of each scale. With forceps 

 remove a scale and note the scalloped margin which was set in the 

 dermal pocket. Examine a scale mounted in water on a slide, 

 using the low power of the compound microscope. Observe the 

 concentric markings or lines of growth. Knowing that those 

 lines formed during the winter season are closer together than 

 those formed during the summer period, determine the approxi- 

 mate age of the specimen. Note the rows of minute denticles and 

 the film of epidermis upon the exposed portion of the scale. 

 Owing to the toothed border this type is known as a ctenoid 

 scale. 



The external fin rays, also called dermal rays, are formed by 

 the dermis of the skin. Mount one of these in water on a slide 

 and examine under the microscope. Note that the rays are com- 

 posed of numerous small joints and that they branch toward the 

 free end. Are the spiny fin rays jointed or branched? 



On the plate illustrating the structural features of the perch 

 label the nostrils, the branchiostegal membrane, a gill arch, 

 indicating its numerical position, a gill slit, raker, and gill. 

 Also the lateral line, fins, the dermal fin rays, the anus, and the 

 urogenital depression. 



