128 LECTUBES ON BIOLOGY 



shape. This structure is probably an echo of ages long ago 

 when they were fixed animals, as the beautiful feather-stars 

 Pentacrinus and Khizocrinus are to-day. For it is a pheno- 

 menon universally observed in the animal kingdom that a 

 sedentary mode of life usually leads to a modification into, or 

 at any rate an approach to, the radial type. I need only mention 

 corals, Chsetopod worms (Serpulidse) and the Bryozoa. 



In any large aquarium we may see lying on the sand awk- 

 ward looking sea-cucumbers ; sea-urchins menacingly project 

 their spines, whilst a star-fish is about to force its proboscis-like 

 stomach between the valves of a mussel and slowly devour the 

 living contents (fig. 37). The superficial observer cannot under- 

 stand how these awkward, cumbersome animals, clad in their 

 calcareous armour, can reach the highest point and crag of the 

 artificial rocks, and even ascend, slowly but surely, the smooth 

 glass wall. But observation will supply the solution. The 

 starfish has finished its repast which seems to have stimulated 

 its energy. Slowly it pushes itself along the bottom until it 

 reaches the glass wall. "As it erects its arms, preparatory to 

 ascending the side, we see that scores of soft tube-feet are pro- 

 truded from the central groove of each arm. These become long 

 and tense and their sucker-like terminal discs are pressed against 

 the glass. These are fixed, and towards the attachment the 

 starfish gently lifts itself. By repeating this process it gradually 

 ascends the glass. The protrusion is effected by the internal 

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

 that the contained fluid, flowing back again from the tube-feet 

 to the internal reservoirs, produces a vacuum between the end of 

 the tube-feet and the surface of the glass. 



" On the dorsal surface, between the bases of the arms, there 

 is a complex calcareous sieve through which the sea-water is 

 taken in. Its pores converge into a fine tube, the ' stone-canal ' 

 which, like a complex calcareous filter, extends vertically through 

 the body, and leads into a ring round the mouth. This circum- 

 oral ring gives off nine transparent vesicles and five radial tubes, 

 one for each arm. Each radial vessel lies in the ventral groove 

 of an arm, roofed by the rafter-like vesicles, and gives off 

 internally reservoir-like bladders or ampullae, and externally the 

 tube-feet. The fluid in the system seems to pass from the radial 

 vessels into the tube-feet, and back again into the ampullae. 



