116 ELEMENTARY BIOLOGY. [xi. 



formed by parallel vertical bars, containing pairs 

 of short rods ; the inner face being formed by a 

 meshwork of large vessels, perforated by wide 

 apertures. 



c. Examine with a higher power : the margins of each 

 cleft will be found covered with large active cilia. 



7. The nervous system. 



a. The cerebral ganglia. 



a. These will be found by carefully dissecting 

 away the bases of the labial palps and the 

 integument on the dorsal side of the mouth. 

 They are two in number and each about the 

 size of a pin's head, but somewhat angular in 

 form. 



/3. The commissures connected with the cerebral 

 ganglia are 



A short cord uniting the two ganglia across 

 the middle line over the mouth. 



A cord, the cerebro-pedal commissure, which 

 runs downwards and backwards from each 

 and becomes continuous with that which runs 

 forwards from the pedal ganglion of the same 

 side (7. b. 0). 



A long slender cord which passes directly 

 backwards from each beneath the organ of 

 Bojanus and joins the parieto-splanchnic gan- 

 glia of the same side (7. c.). 



b. The pedal ganglia. 



a. Lay the animal on one side and proceed gently 

 to scrape away the tissues of the foot at about 

 the junction of its anterior with its middle 



