INTRODUCTION. 11 



partially turns the stomach inside ont. The everted edge of the stomach is 

 wrapi)ed round the prey. Soon the bivalve is forced to relax its muscles and 

 allow the valves to gape. The edge of the stomach is then inserted between the 

 valves and applied directly to the soft parts of the prey, which is thus completely 

 digested. When the starfish moves away, nothing but the cleaned shell is left 

 behind. If the bivalve is small it may be completely taken into the stomach, and 

 the empty shell later rejected through the month. It was for a long time a puzzle 

 in what way the bivalve was forced to open. Schiemenz has, however, shown that 

 when the starfish folds itself in umbrella-like form over the prey, it holds on to the 

 substratum by means of the tube-feet of the distal portions of the arms, Avhilst, by 

 means of the tube-feet belonging to the central portions, it drags apart the valves 

 by main force. He has shown experimentally : (1) that whilst a bivalve may be 

 able to resist a sudden pnllof 4000 grammes, it will yield to a pull of 900 grammes 

 long continued; (2) that a starfish can exert a pull of 1350 grammes; (3) that a 

 starfish is unable to open a bivalve unless it is allowed to raise itself into a hump, 

 so that the pull of the central tube-feet is at right angles to the prey. A starfish 

 confined between two glass plates walked about all day carrying Avith it a bivalve 

 which it Avas unable to open." 



In order to aid the pull recent starfishes are provided with suckers at the 

 extremities of the tube-feet. Each tube-foot is also provided with a reservoir, the 

 ampulla, which assists in keeping it distended with water (Text-fig. 2). 



It was formerly believed that a starfish moved by attaching its suckers to a 

 fixed o])ject, and then puUiug upon this as a fixed point. Jennings (35, p. 98) 

 disproved this by observing the method of progression on loose sand. He showed 

 that the sucker merely served to prevent the foot from slipping, and the tube-feet 

 themselves were really more or less rigid levers, which, just as the legs of the 

 higher animals, swung the body forward. Jennings' observation helps us to 

 understand the reason why the very primitive Asterozoa could walk. It is obvious 

 that if they had to Avait until suckers were well developed they must have been too 

 helpless to survive. 



The more detailed observations given in the succeeding paragraphs show that 

 there is considerable complication in structure to withstand strain. The strain 

 comes, not in walking, but in pulling open Lamellibranchiate mollusca. This 

 method of feeding Avas possible only Avhen the anibulacral arch had become 

 sufficiently strong to Avithstand poAverful lateral pull. It has already been 

 suggested (p. 9) that the primitive forms probably passed small particles of food, 

 by means of the tube-feet, to the mouth. Jennings has observed (35, p. 93) that 

 in Recent starfishes " there are tAvo main methods of conveying food to the 

 mouth. Large objects are usually carried by the active bending of the ray beneath 

 the body, till the object is applied to the mouth. . . . Small pieces of flesh 

 are transported in a somewhat different manner. After being carried to the 



