582 



ECHINODERMATA — CRINOIDEA 



are to be fovmd a series of podia in the form of delicate finger- 

 like processes, which serve only for respiration and for producing 

 a current of water, their surfaces, like that of the grooves between 

 them, being covered with powerful cilia. The anus is at the 

 extremity of a little knob called the anal papilla, situated in one 

 of the interradii (Fig. 264, an). 



As in Ophiuroidea, the ectoderm cells have disappeared over 

 the whole surface of the body, except the grooves and the podia, 

 the only trace of their former existence being a cuticle with 

 adherent nuclei. Pedicellariae are unknown in all Pelma- 

 tozoa ; and spines have only been described from one fossil 

 ^^-. ci^ species of Criuoid. 



/ Beneath the cuticle 



is the dermis, having 

 the composition de- 

 scribed in the case 

 of Asterias ruhens ; 

 this on the aboral 

 side of the calyx 

 gives rise to the 

 " patina," consisting 

 of plates, in part 

 movable on one 

 another, in part im- 

 movably fused to- 

 gether. Those visible 

 ^ J from the: outside are 



Fig. 265.— View of Antedon rosacea from aboral surface. (1) the CCntrO-dorsal 

 X 4. c Centro-dorsal; dr, dvrns ; Jl^, Ii\ R^ the ^^^^^^ f^.^j,, ^jjipi, 

 three radial plates of one column ; syz, syzygy. ' 



the cirri spring; (2) 

 five columns of ossicles termed radials (Fig. 2(36, it\ H^, B^); 

 each column consists of three radials, extending from the 

 centro-dorsal to the origin of a pair of arms. The uppermost 

 radial in each column bears two facets for the articulation of 

 these arms. Each arm is supported by a series of " brachial 

 ossicles " (Br). 



It is evident, both from the number of ambulacral grooves 

 and of the columns of radials, that Antedon has only five radii, 

 and each pair of arms must be regarded as having arisen hj the 

 bifurcation of a primitive arm. This is proved to be true by 



