92 



The Echinus or Sea Urchin. 



mouth upwards (the ordinary position for the mouth being down- 

 wards), and this is accomplished by five rows of suckers, each sucker 

 being under the complete control of the animal, for it can make use 

 of one only, or of the whole five sets at once if necessary. Each sucker 

 is situated at the extremity of a highly elastic tube, which separates 

 into two branches at the base, each branch passing through one of the 

 pores or holes of the ambulacral plates and uniting again with its fellow 

 on the inside. There will thus be twice as many pores as suckers ; 

 the method of working these suckers will be explained further on. 



The only other point observable on the exterior of the echinus is 

 a series of minute structures called pedicellarice. Each consists of 

 a stem with three teeth at the extremity, these teeth continually 

 separating and shutting together with a kind of snap. Their use is 

 quite unknown, in fact by some observers they are considered to be 

 parasites, but the question is still unsolved. Should they prove to 

 be really parts of the echinus, their use will probably be that of 

 clearing the suckers of extraneous particles of dirt, which may have 

 become attached to them. 



Such then is a rough sketch of the external appearance of this 

 creature — simple, and yet marvellously complex in certain points, 

 but this is astonishingly increased when we examine the interior. 



On opening the corona, there will (at the proper season of the 

 year) be observed the five bags of eggs, each about the size of a 

 walnut, a very small amount of internal arrangement of stomach, 

 &c, and perhaps the most extraordinarily complicated arrangement 

 of teeth conceivable, and which seems to be utterly disproportioned 

 to the otherwise "low " structure of the animal. From pole to pole 

 extends an intestinal tube, passing twice round the inside of the corona, 

 to which it is attached by threads, corresponding to the mesentery of 

 higher animals. This tube is nowhere expanded into a cavity corres- 

 ponding to a stomach. There is no liver, no true heart, or blood 

 vessels. The nervous system consists of a ring round the gullet, with 

 five small " ganglia " or knots, from which proceed branches to the 

 eye spots, these branches again giving off a few secondary branches. 



The ambulacral system (that by which the animal is enabled to 

 move along), may be thus described. From the madriporiform 



