176 MORPHOLOGY OF INVERTEBRATE TYPES 



smaller ones with curved edges. The blades are provided with 

 special muscles enabling them to open and close. The pedicel- 

 lariae are thus enabled to grasp small particles and serve as pro- 

 tection against small enemies and as an apparatus to keep the 

 surface of the starfish clean and to help in forwarding food to 

 the mouth. 



The body cavity or ccelome is very large and extends to the 

 end of each arm. It is lined with ciliated ccelomic epithelium, 

 the so-called parietal layer. 



Digestive system. The month as already mentioned is sit- 

 uated in the middle of the ventral disc and is surrounded by the 

 peristomial membrane and five pairs of movable spines. It leads 

 into a very short oesophagus which is followed by the immense 

 stomach. The cardiac part of the stomach, i. e., the one nearest the 

 oesophagus, forms five broad cardiac pouches and can be everted 

 to the outside through the mouth, when the animal is feeding, 

 for the purpose of predigesting the food. The digestive fluid 

 used in this process comes from five pairs of digestive glands 

 situated in the arms, each gland suspended by two mesenteries 

 from the roof of the arm. These glands are diverticles of the 

 stomach. The retraction of the cardiac portion is accomplished 

 by five pairs of retractors or muscles arising from the sides of 

 the ambulacral ridges. The pyloric portion of the stomach, i. e., 

 the one nearest the intestine, is considerably smaller than the 

 cardiac one and has the five pyloric cceca just mentioned. The 

 intestine is very short and insignificant and bears five in- 

 testinal cceca. The anus is situated near the middle of the dorsal 

 surface. 



Ambulacral or water-vascular system. From the per- 

 forated madreporic plate which is situated on the dorsal disc 

 between the bivium, a stout, somewhat curved hydrophoric 

 or stone canal runs toward the ventral body wall. Here it 

 enters the ring canal in an interradius which is inclosed in the 

 ossicles surrounding the mouth. Into the ring canal open nine 

 Tiedemann's bodies (there being only one T. body in the inter- 



