232 ECHINODERMA. 



Alimentary System. 



The starfish is fond of young oysters and other bivalves, 

 and may be found with part of its stomach extruded over 

 them. This protrusible or cardiac portion of the stomach is 

 glandular and sacculated, and bulges slightly towards the 

 arms ; it is followed by an upper or pyloric portion, giving 

 off five branches, each of which divides into two large diges- 

 tive caeca, a pair in each arm (Fig 75.) These glands con- 

 tain a yellowish pigment (enterochlorophyll) and secrete 

 tryptic, peptic, and diastatic ferments. From the short 

 tubular intestine between the stomach and the almost central 

 dorsal anus two little outgrowths are given off, perhaps homo- 

 logous with the " respiratory trees " of Holothuroids. Some 

 parts of the food canal are ciliated. 



Body Cavity. 



The coelome is distinct, though not much of it is left 

 unoccupied either in the disc or in the arms. It is lined by 

 ciliated epithelium, and contains a fluid with amoeboid cells. 

 A few of these have a pigment which probably aids in 

 respiration ; others are phagocytes, which get rid of injurious 

 particles through the " skin gills ; " others continue the work 

 of digestion. 



Water Vascular System. 



When we watch a starfish crawling up the side of a rock 

 we see that scores of tube feet are protruded from the ventral 

 groove of each arm, that these become long and tense, and 

 that their sucker-like terminal discs are pressed against the 

 hard surface. There they are fixed, and towards them the 

 starfish is gently lifted. The protrusion is effected by the 

 internal injection of fluid into the tube feet, the fixing is due 

 to the subsequent withdrawal of the water producing a 

 vacuum between the ends of the tube feet and the rock. 



As to the course of the fluid, it is convenient to begin with the madre- 

 poric plate, which lies between the bases of two of the arms (the bivium}. 

 This plate is a complex calcareous sieve, with numerous perforating 

 canals and external pores. It may be compared to the rose of a water- 

 ing pan, but the holes are much more numerous, and lead into small 

 canals which converge into a main ciliated canal. The latter runs down 

 through the body, and is like a complex calcareous filter. It is called 

 the stone canal. 



