138 INVERTEBRATE ZOOLOGY 



a pigment eye. There are no other organs of special sense. The 

 main nerves of the starfish do not lie within the body-cavity, but 

 in the integument, and can thus be seen from the outside. There 

 are, however, in addition to these nerves, other less impor- 

 tant ones which are internal. We have already observed the 

 radial nerves in the median line of the ambulacral grooves ; the 

 ring nerve can also be seen as a slight ridge just beneath 

 the ring canal. 



Exercise 9. Draw a diagram representing the nervous system. 



The circulatory system consists of a very complicated system of 

 tubes and spaces, filled with a blood fluid, none of which can 

 be seen in a dissection, except an organ usually called the heart 

 or axial sinus. This is a tubular sac which will be found beside 

 the stone canal; within it is an elongated glandular organ 

 called the axial organ or ovoid gland. 



Exercise 10. Draw a diagram representing a vertical section of 

 the animal passing through the madreporic plate and the 

 anterior ray (i.e., the middle trivial ray). 



