136 . THE STARFISH 



II. INTERNAL STRUCTURE 



Exercise 3. — The Skeleton. 



(a) Using the dried specimen, remove the aboral body wall from 

 the disk without injuring the madreporite. Remove also the aboral 

 body wall from one of the arms of the trivium. Pick away the dried 

 remains of the internal organs and note the relation of the skeleton 

 to the mouth. Examine the inner oral surface of the opened arm and 

 find the openings through which the tube feet extend to the outside. 

 Do they pass through or between the plates of the skeleton? Dried 

 specimens macerated in caustic potash will demonstrate the nature of 

 the skeleton. Compare the macerated material with the dried star- 

 fish and determine the body regions in which some of the types of 

 isolated plates were located before maceration. Cut an arm of the 

 dried specimen in transverse section and review the features already 

 observed. Using a sharp scalpel make several sections of one arm of 

 the formalin specimen. How thick is the body wall in the regions on 

 the aboral surface where the skeleton is absent? Compare these sec- 

 tions with Fig. 63. 



Exercise 4. — The Digestive System. 



(b) Using the formalin specimen, remove the aboral portion of the 

 body wall from the middle arm of the trivium, exposing the spacious 

 ccelom. First cut off the extreme tip of the arm and then with strong 

 scissors cut toward the disk on each side, taking care not to injure any 

 internal organ. The dissection should be made under water. As the 

 cut proceeds, the aboral wall may be lifted, and the mesenteries, by 

 means of which the greenish masses of the pyloric caeca, or digestive 

 glands, are suspended from the aboral body wall, may be removed 

 without injury to these organs. What is the nature of the mesenteries? 

 Examine them in the transverse section of the arm previously made. 

 Finally remove the aboral body wall of the arm by cutting across it 

 near the disk. Cut around the margin of the disk above the arms 

 until the cut is near the madreporite. Cut around the madreporite, 

 being careful not to injure the internal structures, and continue the 

 cut until the aboral surface of the disk is free from the remainder of 

 the body wall. Carefully lift up the margin of this circular piece of 

 the body wall so that you do not tear away its attachment to under- 

 lying structures. Determine the nature and extent of the pyloric 

 cffica, and the way in which they arc attached to the remainder of the 

 digestive system. Observe that the saclikc stomach is attached to the 

 center of the disk by means of a short intestine from which the anus 



