154 INVERTEBRATE ZOOLOGY 
The respiratory and exeretory systems are not represented 
by special organs. The ambulacral feet may possibly perform 
both functions. 
The nervous system cannot be studied in a dissection. It is 
essentially like that of the starfish. From the ring nerve 
around the cesophagus five radial nerves pass along the median 
line of the rays to the ocular plates, where each terminates 
in a tentacle. The entire system is within the body-cavity 
instead of a portion of it being outside as in the starfish. 
The circulatory system cannot be seen in a dissection. As in 
the starfish it consists of a complicated system of tubes and 
spaces which contain a colorless blood fluid. An organ of 
problematical function called the heart or axial sinus is found 
attached to the upper end of the stone canal; within it is a 
glandular organ called the ovoid gland or axial organ. 
