THE CYSTIDEA- 



45 



passing up into the suture lines and into the ends of the canals, smooth 

 on its inner surface. Doubt has been cast on the existence of these three 

 layers, but examination of well-preserved specimens suggests the follow- 

 ing interpretation : (TO) is homologous with the " mesostereom " of all 

 Echinoderms, its plates were deposited in a stroma of connective tissue, 

 and presented not only large meshes, but continuous larger canals for 

 the passage of strands of stroma, and perhaps of lacunar blood-vessels : 

 the stroma was thus continuous throughout each plate, and strands often 

 passed over the outer surface, uniting the larger strands and sometimes 

 producing the grooves bent in horse-shoe, as at 2 in Fig. I. ; (e) is not 



FIG. III. 



Section of test in Aristocystis, in the region 

 of a suture (s). Shows epistereom (e), meso- 

 stereom (i)> an d hypostereom (h). m is pierced 

 by haplopores which are joined above at 2, 2. 

 (Original diagram, magnified about 10 diam.) 



St .-- 



Fio. IV. 



Calix Sedgwicki, from a recon- 

 struction by Renault ; about 

 nat. size. 



FIG. V. 



A transverse section of 9. plate of Calix Murchisoni, 

 the stroma - strands replaced by infilling matrix, the 

 stereom dissolved away, x 6 diam. (After Rouault.) 



a truly calcified epistereom, but probably represents a hard epidermis ; 

 there is little doubt that some structure did actually cover the outer ends 

 of the canals ; (h) represents the inner layer of the integument, which 

 towards the margins of each plate was often differentiated into elastic or 

 muscular strands, uniting adjacent plates and giving flexibility to the 

 theca ; if calcified, this layer would be homologous with the "hypo- 

 stereom " of many Crinoids, but, as in the case of (e), there is only 

 preserved to us the space which it occupied, filled with subsequent 

 infiltration or with iron oxide precipitated during the decomposition of 

 the organic matter. The primitive features of Aristocystis are then : 

 indefinite shape of theca, indefinite arrangement of plates, undifferentiated 



