7 6 



THE CYSTIDEA 



canal (which may have contained a nerve of the aboral nerve -system). 

 Interambulacrals numerous and irregular, all pierced by diplopores. 

 Mouth lies in a depression (cf. Edrioaster) ; surrounded by five interradial, 

 slightly forked A, apparently continuous with the adambulacrals, and 

 homologous with the five plates similarly situated in Sphaeronis. Posterior 



FIG. XLVII. 



Mesocystis Pusirefskii. 1, general form, restored after Hoffmann and Nikitin. 2, oral sur- 

 face, after Hoffmann, enlarged ; the deltoids are not clearly shown. 3, structure of the adoral 

 end of a food-groove, modified from Jaekel ; the covering-plates removed from the lower right- 

 hand part ; much enlarged. 4, transverse section of a food-groove, after Jaekel. As, anus ; 

 adAmb, adambulacrals or side-plates (s.p) ; cp, covering-plates or ambulacrals ; Br, dotted 

 outlines of brachioles, borne by Br', brachiole-facet ; fg, food-groove ; iAmb, interambulacrals, 

 pierced by diplopores (p) ; M, supposed hydropores, probably only due to a boring parasite ; 

 0, mouth. 



A pierced by hydropore (and ? gonopore). Anal pyramid in upper part of 

 posterior interambulacrum. Aboral surface of theca composed of numerous 

 small plates, but the structure of its central region is still unknown. 



FAMILY 5. GOMPHOCYSTIDAE. Diploporita in which extension of food- 

 collecting surface is provided by the curving of the five main grooves 

 around the theca and not by their prolongation on to brachioles. 



