228 ECHINODERMA. 



of Holothurians (Fig. 103, 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 amceboid 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 raadre- 

 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-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 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 the stone canal 

 has taken the place of one. In many star-fishes there are five or ten 

 little reservoirs (Polian vesicles) opening into the circumoral ring, but in 

 Aslerias miens these are hardly distinguishable 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 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 reservoir on the dorsal or more internal 

 side. ( See Fig. 100. ) 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 ampullae. 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. 



