ECHINODERMS. 279 



captured these terms bend the arms inwards towards the 

 mouth, giving a somewhat basket-like appearance, these 

 are known as 'basket-fish.' The name brittle-stars is due 

 to the fact that in some the arms are very easily broken. 

 A few brittle-stars produce living young. 



CLASS III. CRINOIDEA (SEA-LILIES). 



While all other echinoderms are free throughout their 

 lives, the crinoids are characterized by being fixed to some 

 firm support by a long stalk arising from the aboral surface 

 of the body. In most the stalk persists throughout life, 

 but in a few, after the adult condition is reached, the body 

 separates from the stdk and thereafter follows a free life. 

 From the central disc or calyx radiate the five (usually) 

 branching arms, and these arms and their branches bear 

 small branchlets, r:o that as these animals rest in their 

 ordinary position, the whole forms a funnel-like net with 

 the mouth at the bottom (fig. 94). On the upper (oral) 

 side of all these branches run grooves converging at the 

 mouth (fig. 95), and so any object which falls anywhere 

 on the funnel is brought to the animal as food. The ali- 

 mentary canal runs spirally through the calyx (fig. 95), 

 and the vent is on the oral surface. The stalk, like the 

 calyx, is strengthened by calcareous plates, those of the 

 stalk being disc-like and piled one on another. 



Crinoids, with the exception of the free forms (Comatula), 

 are among the rarities of museums, as they are found only 

 in the deeper seas. In past time, however, they were very 

 abundant, and whole layers of rock in certain localities are 

 made up of their remains. The fossil forms pres-ent a 

 greater variety of shape than do the living representatives. 



