183 ELEMENTARY BIOLOGY. [XIII. 



LABORATORY WORK. 

 A. GENERAL STRUCTURE. 



1. Go over the specific characters given above (p. 157). 



2. The divisions of the body: head, trunk, two pairs 

 of limbs (see p. 152). 



a. The head. 



Somewhat triangular, with the blunted apex 

 turned forwards and passing broadly, without any 

 neck-constriction, into the trunk ; notice the pro- 

 minent eyes with their lids ; the membrana 

 tympani, a part of the integument stretched 

 over a hard ring, placed on each side, behind and 

 somewhat below the eyes ; the two apertures of 

 the nostrils (anterior nares] between the eyes and 

 the end of the snout ; the mouth-opening ; the 

 hard parts felt through the skin on the upper 

 side of the head ; the soft flexible throat. 



Pass a bristle into one of the anterior nares. 

 Make a small opening in one of the tympanic 

 membranes and pass another bristle into it. Now 

 open the mouth widely ; and, if the bristles have 

 been thrust far enough, the end of the former 

 will be seen traversing the posterior nasal open- 

 ing in the roof of the mouth : while the end of 

 the other will appear in the Eustachian recess 

 which lies at the sides of the back of the oral 

 cavity. The fleshy tongue will be seen, with its 

 bifurcated free end turned backwards. Turn it 

 forwards to see the attachment of its base to the 



