INTERNAL ANATOMY OF THE STARFISH 1 



1. Place a preserved, injected Starfish, in its natural position, aboral 

 surface up, in a wax-bottomed dissecting pan and cover with water. 

 Turn the pan so that the two arms of the bivium are pointing away from 

 you, and fasten the animal in place by inserting a pin through the tip of 

 each arm. 



2. A portion of the body wall covering each arm of the trivium may 

 now be carefully removed as follows : (a) Snip off the extreme tip of an 

 arm with the scissors ; (b) insert point of scissors in the end of the arm 

 and carefully cut through the aboral wall about \ inch to the right of the 

 median line ; (c) extend the cut to the central disc, aiming the scissors 

 throughout at the madreporite ; id) make the same cut to the left of the 

 median line ; (e) beginning at the tip of the arm carefully raise the severed 

 portion of the body wall, detaching it from the underlying tissues and 

 leaving them carefully in place ; (/) make a transverse cut through the 

 body wall at the edge of central disc and discard the detached portion of 

 body wall ; (g) separate the cut edges of the arm and hold apart with 

 pins. 



3. Carry out the procedure given in paragraph 2 for each of the two 

 remaining arms of the trivium and then dissect off and discard the body 

 wall of the central disc, excepting the area immediately surrounding the 

 madreporite, thus exposing the underlying stomach. Note the connec- 

 tions of the stomach with (a) the large paired pyloric caeca which 

 extend into each arm. Remove the caeca from one arm and find (b) a 

 pair of small feathery gonads extending into the arm from the central 

 disc. Identify (c) the retractor muscles extending from the wall of 

 the stomach to their attachment to the ambulacral ossicles in the arm. 

 Examine (d) the ampullae of the tube feet which he on either side of the 

 ridge of ossicles in the median line of the arm. 



4. Draw the dissected animal in outline and fill in the detail in the 

 central disc and in two of the arms to show the structures as observed. 



5. Carefully dissect out the stomach after clipping the retractor 

 muscles, taking particular care not to injure the madreporite. Find the 

 stone canal running orally from the madreporite to the mouth region. 

 Try to trace it to the ring canal encircling the mouth. Make a diagram 

 to show the arrangement of these parts. 



1 B. pp. 76-80. 



277 



