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. 276.- 



■Trochocystis bohemieus, viewed from two sides. 

 (After Jaekel.) 



Moutli. 



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 fiat 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 

 a,rms 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 



