STRUCTURE OF A STARFISH. 205 



tip of each arm, and is usually upturned. It consists of 

 numerous little cups, lined by sensitive and pigmented cells, 

 containing clear fluid, covered by the cuticle. The skin is 

 diffusely sensitive. 



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 

 the stomach, which bulges slightly towards the arms is 

 followed by an upper or pyloric portion, giving off five 

 branches, each of which divides into two large digestive 

 caeca for each arm. These glands contain a yellowish pigment 

 (enterochlorophyll) and secrete tryptic, peptic, and diastatic 

 ferments. From the short tubular intestine between the 

 stomach and the central dorsal anus two little outgrowths 

 are given off, which are believed to be homologous with the 

 "respiratory trees" of Holothuroids. Some parts of the 

 food-canal are ciliated. , 



Body-Cavity. — The ccelome 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 the fluid in the cavity 

 contains a few brownish amoeboid cells, the pigment of which 

 probably aids in respiration. 



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 fact that 

 the subsequent withdraw^al of the water produces a vacuum 

 between the ends of the tube-feet and the surface of the 

 rock. 



But it is necessary to inquire into the course of the fluid. 

 It is most convenient to begin with the madreporic plate 

 lying 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-pan, but the holes are much more 

 numerous and lead into small canals which gradually con- 

 verge into a main canal. The latter runs down through the 



