132 



ECHINODERMATA 



Fig. 56. Ambulacral plates and pores. 

 (W. J. Moore.) 



In the spaces between the ossicles are a number of minute pores , 

 the dermal pores^ bearing retractive dermal branchiae or papulae which 

 are soft filiform processes and concerned in respiration. The body 

 wall is covered with a layer of ciliated epithelium, the epidermis, 

 continued over tubercles, spines, pedicellariae, dermal branchiae and 



tube feet. (Figure 56.) 



Musculature. — The arms 

 are movable, being supplied 

 with muscle fibers in the body 

 wall. 



Digestive System. — The 

 mouth opens through a short 

 passage, the esophagus, into a 

 wide sac, the cardiac divi- 

 sion of the stomach. This is 

 five-lobed (pentagonal), with 

 each lobe opposite one of the five arms. The cardiac stomach is 

 everted through the mouth. Its retraction is effected by special 

 retractor muscles attached at the sides of the ambulacral ridges. 



The cardiac stomach communicates with the smaller pentagonal 

 pyloric stomach, which in turn opens into a short conical intestine, 

 leading upward to open at the anal aperture on the aboral side of the 

 disc of the starfish. The pyloric stomach is extended at its five 

 corners to form a pair of pyloric ceca in each ray. Each pair of 

 pyloric ceca begins as a cylindrical duct, leading into the pyloric 

 chamber. This bifurcates to form two smaller ducts, which give off 

 laterally short branches, each connected with many small glandular 

 pouches. The glandular pouches secrete juices containing enzymes 

 and pass them through the series of ducts into the pyloric stomach. 

 The pyloric (hepatic) ceca are productive of a digestive juice 

 similar to the pancreatic juice of vertebrates, which converts starch 

 into sugar, proteins to peptones and emulsifies fats. Intestinal 

 ceca, attached to the intestine, secrete a brownish material, probably 

 excretory. 



Water Vascular System. — This remarkable system (Figure 57) is 

 used in the starfish for locomotion and in securing its food. It is a 

 specialized portion of the coelom. From the madreporic plate, the 

 stone canal leads downwards to the ring canal or circular canal. (The 

 circular canal bears four pairs of Tiedemann's vesicles and one extra 

 opposite the stone canal); from this canal five radial canals pass out. 



