144 INVERTEBRATE ZOOLOGY 



The respiratory and excretory 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 oesophagus five radial nerves pass along the median 

 line of the rays to the ocular plates, where they terminate in 

 pigment-eyes. 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. 



