CLASSIFICATION ASTEROIDEA 431 



arm towards its tip, termed the " ambulacral " groove. Projecting 

 from the ambulacral groove are found the podia or tube-feet, the 

 organs of movement and sensation of the animal. 



(2) Ophiuroidea (Brittle Stars). Eleutherozoa, in which the 

 body consists of a round disc with long worm-like arms inserted 

 in grooves on its under surface. No anus is present, and the 

 ambulacral grooves are represented by closed canals. The podia 

 are merely sensory and respiratory, locomotion being effected by 

 muscular jerks of the arms. 



(3) Echinoidea (Sea-urchins). Globular or disc- shaped 

 Eleutherozoa, in which the skeleton forms a compact cuirass 

 except for a short distance round the mouth (peristome) and 

 round the anus (periproct). The ambulacral grooves are 

 represented by canals which, like meridians of longitude on a 

 school-globe, run from the neighbourhood of the mouth to near 

 the aboral pole of the body. The spines are large and movably 

 articulated with the plates. The animals move by means of 

 podia and spines, or by means of the latter only. The anus is 

 usually situated at the aboral pole, but is sometimes displaced 

 towards the side, or even on to the ventral surface. 



(4) Holothuroidea (Sea - cucumbers). Sausage - shaped 

 Eleutherozoa, in which the skeleton is represented only by 

 isolated nodules of calcium carbonate, and in which the body- 

 wall is highly muscular. The mouth and anus are situated at 

 opposite ends of the body, and the ambulacral grooves (repre- 

 sented by closed canals) run from near the mouth to the 

 proximity of the anus. Movement is accomplished by means of 

 the podia, aided by worm-like contractions of the body. 



CLASS I. ASTEKOIDEA 2 (Starfish) 



The Starfish derive their name from their resemblance in 

 shape to the conventional image of a star. The body consists 

 of broad triangular arms (generally five in number) which 

 coalesce in the centre to form a disc. The skin is soft and 



1 Gr. aarrip, a " star " ; eldos, " form." Linnaeus established the genus Asterias in 

 1766. Johannes Muller in 1842 used the name "Asteriden," and in System der 

 Asteriden, 1842, by Muller and Troschel, the foundation of our knowledge of the 

 group was laid. 



