ACANTHASCUS CACTUS. 151 



reduced to nothing. In them the terminals are comparatively 

 strong, reaching up to nearly 5 /^ in thickness at base ; they are 

 obsoletely rough all over. The roughness is especially pronounced 

 at their base and here it may often be distinctly observed, under 

 a very high power of the microscope, that it is caused by reverted 

 microtubercles or minute barbs. Each principal may bear two, 

 seldom three or four, divergent terminals, thus giving in all 12 

 or more than 12 terminal points to the entire oxyhexaster. 

 More usuall}'' the oxyhexasters seem to be hemihexactinose and 

 not seldom quite hexactinose (figs. 30 and 31). In the former 

 case, the principals are biterminal if not uniterminal. Thus, 

 forms with 11, 10, 9, 8, 7, or only 6 terminal points are not 

 uncommon. 



Of the oxyhexasters situated in the subdermal space, many 

 (not all) are considerably different in appearance from those 

 described above and which are more deeply situated in the 

 wall. In them the principals are appreciably longer and more 

 slender, while the terminals are simply more slender ; moreover, 

 there occur as a rule two or three terminals to each of the six 

 principals. One specimen of such peripheral oxyhexasters is to 

 be seen in PI. XL, fig. 18, on the right hand side. A com- 

 parison of that oxyhexaster with the others shown in PI. XII. 

 will at once make apparent the differences above indicated. 



In all the oxyhexaster varieties the central cross can easily 

 be brought into view and the extent of its arms exactly deter- 

 mined, if proper steps be taken in preparing. 



PL XII., fig. 33 represents a case of what seems to be an 

 abnormally developed oxyhexaster. It was met with but once in 

 the dermal membrane of a certain specimen and may possibly 

 have been only an extrinsic object. Central node irregularly 



