148 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. 
Bzxercise 10. Draw a diagram representing a vertical section of 
the animal passing through the madreporic plate and the 
anterior ray (t.¢., the middle trivial ray). 
