STRUCTURE OF THE STARFISH. 71 



Kadiating from it are live radial nerves, which lie below 

 the radiating water-tubes, and which may be traced to the 

 tips of the rays, where they will be found to end in small 

 spots of dark-red pigment, the five eye-spots, which are 

 on the odd ambulacra at the ends of the rays. 



VII. Dissect out the stone-canal and notice that it lies in 

 a membraneous pouch, the pericardium, which is formed 

 by two folds, along the inner edge of the inter-radial par- 

 tition. Notice that the lower end of the stone-canal con- 

 nects with the circum-oral water-tube, while its upper end 

 joins the inner surface of the madreporic body. 



VIII. Notice the Jteart, a membraneous pouch which runs 

 alongside the stone-canal. In a living specimen the heart 

 may be seen to pulsate, and when removed and examined 

 under the microscope it will be found to be made up of a 

 number of tubular vessels, twisted together. It is very 

 difficult to trace the course of the blood-vessels, except by 

 the examination of microscopic sections along their course ; 

 but they lie in large chambers, the peri-luemal vessels, 

 and these may be injected, through the pericardium, with 

 which they communicate, and when filled with coloring 

 matter, they mark out the course of the true blood-vessels 

 with sufficient exactness. A fresh specimen should, if 

 possible, be used in making the injection. With a large 

 needle drill a hole through the madreporic body, passing 

 obliquely backwards and downwards from in front, so as 

 to strike the pericardium on the lower surface of the pos- 

 terior end of the madreporic body. Introduce a small 

 canula into the hole, and filling the injecting-syringe 

 with a colored fluid, such as water colored with carmine 

 or indigo, force the fluid, very gently, into the peri- 

 cardium. 



If an injecting-syringe cannot be procured, a glass tube, 



