ASTEROIDEA. 245 



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. 



When a star-fish is crawling 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 towards them the star-fish is gently lifted. 

 The protrusion is effected by the internal injection of fluid 

 into the tube-feet ; the fixing is due to the subsequent with- 

 drawal 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 

 bimum). 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 main ciliated canal, the stone 

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



From it are given oft 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 star- 

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

 the circumoral ring, but in Asterias rubciis these are hardly distinguish- 

 able from the first ampulla.' of the radial vessels. These run along the 

 arms, and lie in the ambulacra! groove beneath the shelter of the 

 rafter-like ossicles. From them branches are given oft* to the bases of 

 the tube-feet, but from each of these bases a canal ascends between 

 each pair of ambulacral ossicles, and expands into an ampulla or reser- 

 voir on the dorsal or more internal side (see Fig. 117)- The fluid in 

 the system may pass from the radial vessels into the tube-feet, and 

 from the tube-feet it can flow back, not into the radial vessel, but into 

 the ampullie. There are muscles on the walls of the tube-feet, ampullae, 

 and vessels. At the end of each arm there is a long unpaired tube- 

 foot, which seems to act as a tactile tentacle, and has also olfactory 

 significance. 



With regard to the vascular system there is considerable uncertainty. 

 It is well developed in certain Echinoderms, although there is no 

 heart, but has not yet been properly worked out in Asteroids or 

 Ophiuroids. There is a " pseud-hremal sinus" surrounding the stone 

 canal, leading into a circum-cesophageal ring, which gives off a vessel 

 along each ray. 



