70 II03IES HN'DEE THE SEA. 



The mouth having been mentioned in connection with 

 the ambulacra, we will examine its structure. 



Judging by that of the Starfishes, to which the Urchins 

 are so closely allied, we might naturally expect to see a 

 similar structure. But, in the Echini, we find a form of 

 mouth so widely distinct from that of the Starfishes, 

 that it seems as if it belonged to a totally distinct group 

 of animals. 



As the reader may remember, the mouth of the Star- 

 fish is simply an aperture at the junction of the rays, 

 without either teeth or jaws. Through this aperture are 

 protruded certain membranous vesicles, which have the 

 power of sucking an oyster or other bivalve from its 

 shell. 



The Echinus would not be able to feed after this 

 fashion. It has no flexible rays wherewith to grasp its 

 prey, and the mouth is therefore constructed on a totally 

 different principle. It is not easy to give a full descrip- 

 tion of this wonderful apparatus without more diagrams 

 than can be accommodated within our limits. 



If the reader Avill refer to Fig. 4, he will see a conical 

 projection beloAv, formed of several pointed pieces. In 

 Fig. 5, where the Echinus is shown in section, the same 

 projection is shown, together with its continuation into 

 the interior. Altogether, there are five of these pieces, 

 corresponding, like the ambulacral apertures, with the five 

 rays. Fig. 5 shows three of these rays, with the aper- 

 tures slightly indicated. 



Again referring to Fig. 4, the reader will see that the 



