ECHINODERMATA 



same side as the mouth. Ambulacral grooves are lack- 

 ing and the tube feet are rudimentary, locomotion being 

 effected by movements of the very flexible and muscular 

 arms. 



The brittle Sfor^Qfikiurd) is common along the Atlan- 

 tic coast. Astrophyton, the " basket fish," has rays which 

 are very much branched. 



CLASS 3. Echinoidea 



These are free echinoderms with a globular or disk- 

 shaped shell composed of closely-joined calcareous plates. 

 There are no ambulacral grooves, the tube feet projecting 

 through openings in the plates arranged either along 

 meridional lines or in the form of a star-shaped rosette. 



The sea urchin is encased in a thin, hollow, spherical 

 shell covered with spines. 15 The mouth is underneath, 

 and contains a 

 dental apparatus 

 more complicated 

 than that of any 

 other creature. It 

 leads to a diges- 

 tive tube, which 

 extends spirally 

 to the summit of 

 the body. The 

 spines are for bur- 

 rowing and loco- 

 motion, and are 

 moved by small 



mncr>1^o <= \\ K 

 JSC ieS, C 



ing articulated by 



ball-and-socket j'oint to a distinct tubercle. When stripped 

 of its spines, the shell (or "test") is seen to be formed 

 of a multitude of pentagonal plates, fitted together like a 



4^' ~ Under surface of a Sea Urchin (Echinus escu- 

 lentus) , showing the mouth, tips of the teeth, and rows 

 of suckers among the spines. British seas. 



