IX 



PHYLUM ECHINODERMATA 



391 



Around the mouth is a whorl of tentacles pinnate, shield-shaped, or 

 arborescent. The tube-feet are sometimes entirely absent. When 

 present they are usually uniform in character throughout, and 

 may be arranged in five regular longitudinal rows, or scattered 

 over the entire surface. Sometimes, as has already been stated in 

 the account of Colochirus, the tube-feet of the dorsal surface and 

 even some of those of the ventral may assume the form of papilla?. 

 In the Elasipoda the tube-feet of the dorsal surface are remarkably 

 modified, taking the form of greatly elongated processes. 



In the Crinoidea the general shape is that which has been 

 described in the case of the feather-star star-like, with a central 



I 



Kic;. 313. Antedon. Side view of entire animal. (From Leuckart and Nitsche's wall- 

 diagrams.) 



disc and a series of radiating arms, which usually branch dicho-- 

 tomously. In the stalked forms (Fig. 314) a stalk, consisting of 

 a row of elongated ossicles connected together by bundles of 

 ligamentous fibres, attaches the animal to the sea-bottom. Along- 

 some of the joints of the stalk are usually arranged a number 

 of slender, many-jointed appendages the cirri. At its base the 

 stalk usually breaks up into a number of root-like processes ; distally 

 it becomes continuous with the central disc. The ossicles forming 

 the skeleton of the central disc are tlae Nasals and the radials: with 

 the latter articulate externally the brachials, a single row of which 

 gives support to each of the arms and its branches, while similar rows 

 of smaller ossicles support the pinnules the lateral appendages 

 which fringe the arms in a double row. In the free forms (Feather- 

 stars) the stalk is absent in the adult condition, though present 



