196 ELEMENTARY BIOLOGY. [CHAP. 



cells, occurs in the integument of the frontal region of the 

 head. 



Cells containing pigment abound in the integument and 

 undergo remarkable changes of form, the pigment being 

 sometimes drawn together into a spheroidal mass at other 

 times distributed in a radiating fashion. 



LABORATORY WORK. 

 A. GENERAL STRUCTURE. 



1. Go over the specific characters given above (p. 164). 



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

 limbs (see p. 159). 



a. The head. 



Somewhat triangular, with the blunted apex 

 turned forwards and passing broadly, without 

 any neck-constriction, into the trunk; notice 

 the prominent 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 

 opening in the roof of the mouth : while the end 

 of the other will appear in the Eustachian recess 



