STRUCTURE OF THE STARFISH. 65 



of the pentagon are opposite the axes of the rays, and the 

 hepatic ducts ^pen into the ab-oral surface of the sac, a 

 little above the angles. 



d. Near the centre of the ab-oral surface of the pyloric 

 sac, notice the short, cone-shaped intestine. The anus is 

 so small that it is very difficult to find. It is a little to 

 the left of an imaginary line drawn from the madreporic 

 body to the tip of the anterior ray. A line drawn from 

 the centre of the disc to the angle between the left ray of 

 the bivium and the left ray of the trivium passes through 

 the anus. 



e. At or near its union with the pyloric sac the intestine 

 is joined, upon its left side, b}' a small duct Avhich leads to 

 a light-colored arborescent pouch ; the respiratory tree 

 (Fig. 30, g). This pouch is divided into two sacculi, 

 and these again into small ampullae. Each of the primary 

 divisions of the organ lies opposite an inter-radius, or 

 angle (Fig. 30, d) between two rays. One is opposite 

 the inter-radius between the left ray of the trivium and the 

 left ray of the bivium, and the other betweeen the left ray 

 of the trivium and the anterior ray. 



f. Radiating from near the centre of the disc are the 

 five extensor muscles (Fig. 30, n) of the rays. Normally 

 these are attached to the alvoral integument. They i-un 

 from near the centre of the disc to the tip of each i-a^^ 

 giving off, on each side, lateral fibres which are attached 

 to the ab-oral ossicles. By the contraction of these 

 muscles the free ends of the rays may be bent upAvards 

 or from side to side. 



g. Cut along the sides of one of the unopened i^ays from 

 base to tip, and also cut through the ab-oral surface of the 

 disc at the base of the ray, and through the hepatic duct, 

 so that the integument, with the organs attached to it, can 

 be turned out from the centre ; notice : — 



