CHAPTER XX 



ECHINODERMATA {CONTINUED) : PELMATOZOA CRINOIDEA = SEA- 

 LILIES THECOIDEA CARPOIDEA CYSTOIDEA BLASTOIDEA 



SUB-PHYLUM II. PELMATOZOA 



The Pelmatozoa differ from the Eleutherozoa in several im- 

 portant respects. They are fixed (at any rate in the young 

 stage) by the centre of the aboral surface, and this portion of the 

 body usually takes on the form of a stem supported by a definite 

 series of ossicles, so that we can discriminate a " calyx " the 

 main part of the body from the " stem." Further, the podia 

 and the ambulacral grooves seem to be always covered with 

 powerful cilia, which are employed in producing a current which 

 sweeps small organisms to the mouth. The podia are never 

 locomotor in function ; their use is similar to that of the ten- 

 tacles on the lophophore of Polyzoa and Brachiopoda. 



The living Pelmatozoa are very few in number compared 

 with the extinct forms. It may with justice be said that the 

 group is nearly extinct ; indeed, out of its five classes one 

 alone, and that the most highly specialised class, survives till 

 the present day. Now we have already seen that, in the 

 case of the Eleutherozoa, if the annectant fossil types were 

 taken into consideration, the definition of the classes would be 

 difficult, so that it is not to be wondered at if the classes of 

 the Pelmatozoa are also somewhat difficult to define ; and it 

 must be added that this difficulty is not only due to the 

 fact that intermediate types occasionally occur, but also to 

 our ignorance of the functions of many structures found in 

 fossil types, speculations regarding which are to be received 

 with caution. Bearing in mind, then, the provisional nature 



579 



