Echinoderms. 



73 



ing them again the animal draws itself along. The water necessary 

 for the expansion of the feet is taken into the body by a special 

 system of canals (which have nothing to do with a blood-system) 

 leading to every one of the feet. 



These feet are equally important to the Sea-urchins to catch 

 their prey. If any animal upon which they can feed comes into 

 their vicinity, they protrude some of these feet till they reach 

 its body. Should it not notice in time the attack which is being 

 made upon it, or not be strong enough to tear the attached sucker- 

 feet, it is lost and, caught like Gulliver by the Liliputians, is slowly 

 drawn into the mouth and then gradually devoured. But, so 

 as not to be recognized too soon in stalking their prey, many 

 Sea-urchins cover themselves on their upper surface with all 

 sorts of stones, shells, and pieces of sea-weed, and are thus more 

 often successful. 



The mouth of the Sea-urchins and Starfishes is on the lower 

 surface of the body, and is therefore only visible when they lie 

 on their back or are attached to the glass on the front of the 

 tank. Many Sea-urchins have five strong teeth of very complex 

 structure; others lie always buried in the sand and swallow it, 

 so as to digest whatever organic substances it contains. The 

 Starfishes have no teeth, but the walls of their alimentary canal 

 secrete such strong juices that they can kill animals which they 

 have caught with their sucker-feet and brought to their mouth. 

 In this manner Asterias, the stomach juices of which contain a 

 strong acid, dissolves parts of the shells of the mussels, snails 

 and sea-urchins which it catches and sucks out the soft parts 

 through the holes thus made. If this Starfish catches a fish which 

 is too large to be swallowed, it turns its stomach inside out, sur- 

 rounds its prey with it and digests it outside the body. For this 

 reason Asterias and Astropecten are both dangerous guests on 

 oyster-beds. 



Like some of the vSea-urchins, most Sea-cucumbers live by 

 swallowing sand and mud and digesting adhering organic particles. 

 Some (e. g. Cucumaria, fig. 9) manage matters very differently. 

 They remain motionless on a stone or other prominent object, 

 stretching out their large branching tentacles and carrying them 

 one after the other into their mouth, sucking off the small animals 

 which in the meantime have settled upon them. With a little 

 patience this proceeding may be easily observed. 



Some Sea-urchins (e. g. Dorocidaris, fig. 7) have very long 

 spines; in these cases, however, the sucker-feet can be extended 

 to great length, for they must always reach beyond the spines. 

 In many cases we find between the spines special moveable stalks 

 bearing small pincers, which can also seize small objects. — The 

 Starfishes have primitive eyes at the ends of the arms. 



