88 Laboratory Guide in Zoology 



III. — Appendages. Study the two fins running along the 

 dorsal median line. They are made up of a web sup- 

 ported by hard spines. How do the spines in the two 

 fins differ .' The tail is called the caudal fin. What 

 kind of rays has it .? When the caudal fin is symmetri- 

 cal, it is said to be homocercal ; when it is bent so that it 

 is unsymmetrical, it is said to be heterocercal. Which is 

 the case in the perch .' On the mid-ventral side of the 

 body, just in front of the caudal fin, find the anal fin. 

 Note the fact that all fins studied above are median or 

 single, i.e. never in pairs. Next observe a pair of fins 

 (pectoral) just back of the head, one on each side of the 

 body. They correspond to and indeed are homologous 

 with the fore legs or arms of the higher animals. Back 

 of the pectoral fins, and nearly on the ventral line, find 

 the pair of pelvic fins, corresponding to the hind legs of 

 the higher animals. Take a general view of the fins 

 with reference to their uses. Tell which keep the ani- 

 mal in an upright position, and which give it propelling 

 power and guidance. 



IV. — Other Organs. Study the action of the jaws, noticing 

 that both have movement. Is the action up and down 

 or from side to side .' Place a finger in the mouth to 

 feel the teeth, noticing their slanting direction. Study 

 the size and position of the eyes. Find with the for- 

 ceps the bony socket encasing the eyes. Move the 

 eye itself to find its field of vision. Are there any 

 eyelids 1 Find the nasal opening in the upper lip. 

 Find the anal opening just in front of the anal fin. 



V. — Breathing Apparatus. Find a large, slit-like opening 



on each side of the head just back of the mouth which 

 communicates directly with the mouth cavity. These 

 are the gill-chambers, and the flaps closing over them 



