ECHINODERMA'IA. 17S 



be seen to be fitted on by a sort of ball and socket 

 jointj of wbicli the ball is left behind as a tubercle, 

 if the spine is palled ofiF. In some Sea-urchins 

 (Gidaris) there are only a few spines, and those very 

 large compared with the size of the urchin. The 

 urchin is enclosed in a calcareous box, composed of 

 numberless, small plates. In the common urchin, 

 five interlapping double rows of these correspond 

 with the five ambulacral grooves of the starfish, with 

 their opposite ossicles : while between are five other 

 interlapping double rows inter-radial in position ; 

 these pass from the apex to the mouth, which is 

 exactly in the centre of the lower surface. The 

 former contain tube-feet, • which pass through very 

 minute pores in the shell (easily seen when the shell 

 is dry), and spines also; while the latter are covered 

 with spines only. Sea-urchins have also other 

 external appendages, called pedicellarise, modified 

 spines, which are like a bird's beak on a stalk, and 

 keep snapping constantly, and the function of which 

 is probably to remove waste particles. The common 

 Sea-urchin has a very complicated apparatus of teeth ; 

 these are in the form of a five-partite cone, the five 

 sharp ends peeping out as sharp teeth, while on dis- 

 secting the apparatus each is found to be attached to 

 a long comb-like saw. In Gidaris, Echinus, and their 

 allies, the shell is so nearly round as to be described 

 as regular, while the ambulacral areas are said to be 

 perfect, i.e., they run uninterruptedly fi-om the apex 

 to the mouth. In the other forms the ambulacral 



