16 LABORATORY WORK. 



animal without bilateral symmetry? Make out the re- 

 gions, head, trunk, and tail. Is there a neck? Where is 

 the vent? 



How many fins do you find? How many are paired 

 and how many unpaired? Are any in the median line 

 of the body? Can the fins be regarded as folds of the 

 skin? The various fins have names. The median fins 

 are the dorsal on the back, the anal on the ventral surface 

 just behind the vent, and the caudal at the end of the tail. 

 Do you find a skeleton in any of these? Is the caudal fin 

 homocercal (with equal lobes) or heterocercal (with unequal 

 lobes)? The paired fins are, in front, the pectorals; behind, 

 the ventrals. Do these compare, in position, with your 

 own limbs? 



How many eyes are there, and where are they placed? 

 Are they movable? Are eyelids present? Notice in 

 each eye the colored iris around the black pupil* 



How many nostrils and where are they? Examine 

 carefully and see how the opening is subdivided by a 

 flap of skin. Sketch. Probe with a bristle and see if 

 the nostrils connect with the mouth or throat. How 

 does this compare with what occurs in yourself? 



Where is the mouth? Is there a tongue? Do you 

 find any ears? Behind the eyes are a pair of openings, 

 the spiracles; probe and see if they communicate with 

 the throat. On the sides of the neck, in front of the 

 pectoral fins, are the gill-slits or branchial clefts. How many 

 are there? 



Draw the fish from the side, natural size, inserting and 

 naming all the parts made out. 



* In preserved ' pups ' the lens may be whitened so that the 

 pupil appears white instead of black. Colors also fade in alcohol or 

 formol. 



