CYSTOIDEA 



597 



some cases slight horn-like outgrowths of some of the plates of 

 the calyx may support prolongations of the grooves. - 



This group, which, like the foregoing, commences in the 

 Cambrian, is perhaps more primitive than the Thecoidea in 

 showing less influence of the water-vascular system on the 



Fig. 



-Trochocystis bohemicus, viewed from two sides, o, Mouth. 

 (After Jaekel.) 



skeleton ; but in the presence of a differentiated stem and the 

 development of only two rays, it is more differentiated. The 

 anus is on one of the flat sides, covered with a flat plate acting 

 as a valve. The members of this group were formerly confounded 

 with Cystoidea, from which they differ in the absence of the 

 characteristic pores. Trochocystis, the genus figured, is devoid of 

 any horn-like outgrowths of the calyx. 



CLASS IV. CYSTOIDEA 



Pelmatozoa with respiratory organs in the form of " diplo- 

 pores " or " pore-rhombs." In a great many cases there is 

 a stalk, but in other cases this is atrophied, and the animal is 

 attached by the base of the calyx. The radial canals run for a 

 shorter or longer distance over the calyx, but the plates of the 

 calyx themselves are not modified for them. Either they run in 

 simple grooves, or they are protected by a special series of plates 

 lying above the plates of the calyx. The terminal portions of 

 the radial canals are in all cases free, supported by unbranched 

 arms consisting usually of a double row of ossicles. These arms 

 are termed " fingers." 



It will be gathered from the description just given that the 

 fingers and the respiratory organs distinguish Cystoidea from 



