596 



ECHINODERMATA THECOIDEA CARPOIDEA 



ment of an enormous anal tube, so large that in extreme 

 cases (Uucalyptocrinus) the arms may be lodged in grooves 

 of it. 



CLASS II. THECOIDEA (EDEIOASTEROIDEA, Bather) 



These remarkable Pelmatozoa are the most primitive known. 

 They have sac - like or sometimes cushion - shaped or even 

 disc - shaped bodies, covered with numerous irregular plates 

 without any symmetry in their arrangement. There is no 

 stem, but when they are fixed this is effected by an adhesion 

 of the aboral pole. There are no arms, but on the upper 

 surface is to be seen the impression of five ambulacral grooves 

 ^ radiating from a central mouth. 



These grooves are bordered by 

 ' covering plates, which in the 

 earliest form (Stromacystis) are 

 " seen to be slight modifications of 

 the plates covering the upper sur- 

 face of the body, but in the later 

 genera (Kg- 275, Thecocystis) 

 become specialised. The anus is 

 situated on the side, as is also 

 the madreporite. It has been 

 suggested that Eleutherozoa were 

 X 6. derived from this group ; that in- 

 madre- (Jiyiduals Were occasionally over- 

 turned by the waves or currents,, 

 and in this way compelled to use their podia for locomotion. 

 When Eleutherozoa, however, have a fixed stage in their develop- 

 ment, they are fixed by the oral, not the aboral, surface, and hence 

 can have no close affinity to Thecoidea. Thecoidea begin in the 

 Middle Cambrian, but according to Jaekel impressions in the 

 Lower Cambrian, referred to Medusae, may be casts of this group. 



Fig. 275. — Thecocystis saeculus. 

 a. Anus ; m, mouth ; p, 

 porite(?). (After Jaekel.) 



CLASS IIL CARPOIDEA 



Pelmatozoa with a well -developed stem; body bilaterally 

 compressed ; only two rays apparently developed. These are 

 indicated only by grooves radiating from the mouth ; but in 



