26 ZOOTOMY. 



the nasal sac, and behind ending blindly a little posterior to 



the first internal gill-cleft. 



XIX. If another specimen is available, make a series 

 of transverse vertical sections through the entire 

 animal previously hardened in alcohol or chromic 

 acid (see Figs. 68). The sections should be 

 about 2 to 3 mm. thick in P. fluviatilis, 8 to 

 10 mm. in P. marinus. Be careful not to dis- 

 turb the order of the sections. Examine each one 

 separately, both from its anterior and its posterior 

 face, and observe the precise relations of the various 

 structures already seen by dissection. 1 



1 It is important not to omit this section unless time or material fails, 

 as it forms an excellent introduction to the art of interpreting sections. 



