STRUCTURE OF STARFISH 295 



lated, and bulges slightly towards the arms ; it is followed 

 by an upper portion, giving off five branches, each of which 

 divides into two large digestive caeca — a pair in each arm 

 (Fig. 159). These glands are comparable to a pancreas ; 

 their secretion contains three ferments, which convert 

 proteins into peptones, starch into sugar, and break up fats 

 into fatty acids and glycerine. From the short tubular 

 intestine between the stomach and the almost central 

 dorsal anus two little outgrowths are given off, perhaps 

 homologous with the " respiratory trees " of Holo- 

 thurians (Fig. 167, r.t.). Some parts of the food canal are 

 ciliated. 



The ccelom 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 con- 

 tinue the work of digestion. 



When a starfish is crawUng up the side of a rock, scores 

 of tube-feet are protruded from the ventral groove of each 

 arm ; these become long and tense, and their sucker-like 

 terminal discs are pressed against the hard surface. There 

 they are fixed, and pull up the starfish by muscular contrac- 

 tion. The protrusion is effected by the internal injection 

 of fluid into the tube-feet ; the fixing is due to the pro- 

 duction of 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 

 madreporic plate, which Hes 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 watering-can, but the holes are much more numerous, and lead 

 into small canals, which converge into a onain ciliated canal, the stone 

 canal. This, as usual, opens into a ring canal around the mouth. 

 The ring canal gives off nine glandular bodies (Tiedemann's bodies), 

 and five radial tubes, one for each of the arms. Considerations of 

 symmetry suggest that there should be ten glandular bodies, but in the 

 inter-radius containing the stone canal there is only one. In many 

 starfishes there are five or ten little reservoirs (Polian vesicles) opening 

 into the circumoral ring, but in Asterias rubens these are hardly dis- 

 tinguishable from the first ampullae of the radial vessels. These run 

 along the arms, and lie in the ambulacral groove beneath the shelter 

 of the rafter-like ossicles. From them branches are given off to the 



