ECHINODERMATA. CRINOIDEA. 85 



CLASS. CRINOIDEA. 



The Crinoidea includes the sea-lilies or feather-stars. 

 Unlike the star-fishes and sea-urchins, these have the oral 

 surface directed upwards and the dorsal surface down- 

 wards. In very many forms the animal is fixed by means 

 of a stem given off from the dorsal surface, but in others 

 this is only found in the young state, the adult form 

 being free. The skeleton consists of a stem, and of a cup 

 or calyx formed of calcareous plates closely united and 

 arranged in a regular manner; from the margin of this, 

 branches or arms are given off. 



The soft parts consist of a central mass known as the 

 disc, placed in the calyx, and of arms which come off from 

 the disc ; these may be five in number or there may be as 

 many as two hundred produced by repeated branching. 

 The whole of the alimentary canal is situated within the 

 disc, the mouth and anus being on the upper surface. On 

 each side of the arms there is a row of small appendages, 

 which alternate ; these are the pinnules. On the upper 

 surface of each pinnule and of each arm there is a groove, 

 which is continued on to the disc, where they unite, 

 ultimately forming five main channels which meet in the 

 mouth, this being usually central. The grooves are lined 

 with cilia, by the movements of which food particles are 

 conveyed to the mouth. The anus is placed in one of the 

 spaces between the main channels on the disc, usually at 

 the end of a tubular process. In some forms the surface of 

 the disc is covered with small calcareous plates, in others 

 there are only isolated granules. Immediately under the 

 groove of each arm there is a radial nerve cord, these like 

 the grooves unite to form larger trunks at the margin of 

 the disc and ultimately join as a ring round the mouth. 



