32 Invertebrate Zoology. 



Draw the nervous system. 



With a strong knife, make a cross-section of one of the 

 arms. Note the perivisceral cavity and identify the con- 

 tained organs. Observe the arch of the aboral integument, 

 and below, the large furrowed ambulacral ossicles. Where 

 the ambulacral ossicle of the right meets that of the left, do 

 you find the cross-section of the radiating ambulacral water- 

 tube? 



Remove the aboral integument from one of the arms and 

 note that the ambulacral plates form an unbroken ridge. 

 Trace this ridge to the tip of the arm and draw it, some- 

 what enlarged. 



Remove the aboral integument from the body, taking care 

 not to disturb the madreporic plate. Clean the skeleton 

 and note any local specializations of the ambulacral plates. 

 Do you find inter-radial partitions ? 



Draw the skeleton of the body. 



