12 ECHINODERMS OF THE BRITISH ISLES 



less modified, long, whip-like, stiffened or combed, and are termed 

 oral pinnules. These latter especially are of great systematic 

 value owing to the great variation in the number and shape of 

 their joints. In the systematic descriptions the pinnules are 

 designated P^, Po, P3, etc. 



The ambulacral grooves proceed from a ring around the mouth 

 and form a conspicuous five-rayed star on the ventral side of the 

 body (the disk). At the edge of the disk each groove bifurcates ; 

 one branch goes to each arm, along its ventral side, usually to 

 the very tip, sending off a side branch to each pinnule. In 

 the grooves the tube-feet or tentacles are found, arranged in a series 

 along each side of the groove. It is characteristic of the sea-lilies, 

 contrary to all other echinoderms, that the tube-feet are arranged 

 in groups of three. They are set with sensory papillae, but have 

 no sucking disk ; they have not the function of locomotor 

 organs. Along the ambulacral grooves there are some small 

 plates, the side and covering j^lcites, which, in the Comatulids, 

 are generally reduced to small, microscopical spicules, while in 

 the stalked forms they are mostly well developed. 



On each side of the ambulacral groove lies a series of small 

 bodies about the size of a pin-head, the sacculi, the function 

 of which is not yet quite settled. Probably they have some- 

 thing to do with excretion (they have been regarded as parasitic 

 organisms). They are colourless in life, but in preserved Comatu- 

 lids are generally very conspicuous ; in many forms they secrete 

 a colouring matter that colours alcohol intensely red. In the 

 stalked Crinoids {e.g. in Rhizocrinus) they are often of a yellowish 

 colour and difficult to observe. 



The mouth usually lies in the middle of the disk and the anal 

 opening about midway between the mouth and the edge of the 

 disk, generally on a high conical process, the anal cone. In many 

 forms (of the family Comasteridse), however, the anal cone is 

 situated in the centre of the disk, the mouth, w ith the surrounding 

 ambulacral grooves, being pushed out towards the margin. 

 The intestinal canal makes one or (in the Comasteridae) more 

 dextral coils ; it is generally provided with some sac -shaped 

 outgrowths which jDOSsibly have the function of a liver. 



There is no madreporite ; instead, there are in each interradius 

 a number of small pores, the hydropores or calyx pores, which lead 

 into the body cavity, into which open also the numerous small 

 stone canals that issue from the ring canal. Rhizocrinus has 

 only one hydropore and one stone canal in each interradius ; in 

 Antedon there are several hundreds of them. 



