CRTNOIDEA — ANATOMY OF ANTEDON 



581 



animal is 

 the sub- 

 265, cir). 



stalkless forms are comparatively common. We shall select as 

 type for special description the common Feather-star, Antedon 

 rosacea (hijida), which can be dredged in depths of ten fathoms 

 off* the soutli-west coast of England. 



The animal consists of a small flattened calyx, from which 

 radiate out ten long delicate arms, each fringed with a double 

 series of short branches called " pinnules." In the centre of the 

 aboral surface can be seen 

 the centro -dorsal plate 

 (Fig. 265, c), a knob-like 

 .stump of the broken-off 

 stem, covered with small 

 whip - like outgrowths 

 called " cirri," by means 

 of whicli the 

 anchored to 

 stratum (Fig. 

 When Antedon is dis- 

 turbed it relaxes its hold, 

 and swims by graceful 

 muscular movements of 

 the arms. These are 

 arranged in five pairs, 

 and the corresponding 

 members (right and left) 

 of all the pairs are bent 

 and relaxed together. On 

 coming to rest the animal 

 reattaches itself by means 



of the cirri. These are composed of cylindrical ossicles joined 

 to one another by muscles, and they can thus act as efficient 

 grasping organs. In the centre of the oral surface, which 

 is termed the " tegmen," and is soft, flexible, and without 

 visible calcifications, is situated the mouth, surrounded by five 

 short triangular flaps called " oral valves." In the intervals 

 between these valves, grooves radiate from the mouth which 

 bifurcate at the points of origin of each pair of arms, and are 

 continued over their surfaces. These grooves correspond to the 

 ainbulacral grooves of Asteroidea, and to the epineural canals of 

 the other classes of Eleutherozoa. At each side of each '^roove 



Fifi. 264. — Oral view of Antedon rosacea. x 3. 

 «, Ann ; a.g, ambulacral groove ; an, anus ; c, 

 calyx ; m, mouth ; p, piiniules. 



