Branch Echinodermata 37 



of the groove. See if you can find there a small, 

 white thread-like structure — a nerve trunk — sending 

 out branches to the feet. Follow this to the end of the 

 ray, where it ends in the midst of a little bunch of 

 spines. Look at this bunch of spines carefully with a 

 pocket lens. Examine the termination of each ray in 

 this way. Is there any connection of the nerve trunk 

 with the red spot at the end of the arm .■' This spot is 

 sensitive to light, and for that reason is called the eye- 

 spot. Follow the nerve trunk to its inner termination 

 at the base of the ray. Is it connected with the circu- 

 ' lar nerve ring, surrounding the peristome and the 

 mouth ? 



INTERNAL ANATOMY 



X. — With sharp-pointed scissors, remove the integument 

 from the dorsal surface of one of the rays, being very 

 careful not to disturb any of the underlying parts. 

 Remove also the body wall from the main dorsal sur- 

 face, leaving the madreporite untouched. Be espe- 

 cially careful to harm none of the internal organs when 

 removing this part of the body wall. 



XL — Observe the following structure : Two long pointed, 

 brown, lobulated bodies lie side by side, nearly filling 

 the ray from base to tip. These are digestive glands 

 (that is, organs the cells of which secrete a digestive 

 fluid) ; they are called hepatic coeca. 



XII. — Notice the colorless ducts running along the centre 

 of these masses, uniting near the bases of the ray. 

 Follow up this common duct, and see that it empties 

 into a flat, membranous, pentagonal-shaped sac, a part 

 of the stomach called the pyloric sac. Below this is 

 the stomach proper, with which the pyloric sac is con- 

 tinuous. 



