ACANTHASCIN^.. 135 



liold it not impossible that their development may under cir- 

 cumstances be quite sujipressed, in which case the limiting layer 

 alone would stand for the basidictyonal plate. 



Finally with respect to the hexasters, it may be said that 

 there are three forms occurring together in a. species ; viz. oxy- 

 hexasters, discoctasters and microdiscohexasters. 



The oxyhexasters are the most al)undant of all. Strong and 

 wide-spread is the tendency shown by them to assume hemi- 

 hexactinose and quite hexactinose forms. In some species the 

 hemihexactinose form predominates; in some others, the hexactinose. 

 In 11. tenuis (F. E. Sch.), as before alluded to, all the oxy- 

 hexasters present appear to be hexactinose. Basing our description 

 on normally developed oxyhexasters, the principals are always 

 very short, — often so exceedingly short as to be called vestigial. 

 The number of terminals most frequently borne by a principal 

 is two f' but it may sometimes l)e three and rarely four. The 

 terminals are nearly smooth or more frequently rough. The 

 roughness may be developed on their l)asal parts into retroverted 

 prickles or barbs. In several sjiecies the oxyhexasters situated in 

 the periphery of the sponge-wall, but particularly in the subdermal 



* For Ihjse oxyhexasters generally in which tlie principals appear bifurcated in that 

 tliey are provided eacli with two terminals, it has been given by F. E. Schulze ('97 a) 

 as an ajjproxiniate rule that the plane of bifurcation of a principal stands at right angles 

 with that of another belonging to the same axis, and tliat the six separate bifurcation 

 planes in one oxyhexaster of tlie kind correspond to tlie so-c.illed secondary planes of 

 synnnotry in the isometric crystal system, the primary or principal planes of symmetry 

 being given by the principals forming the three axes. So that the princijials and the 

 terminal fork-; should represent all the nine possible planes of symmetry distinguishable 

 in a regular cry.-tal (thre? principal planes determined by the axes and six secondary 

 planes determined by diagonally opposite edges of a cube). I liave not specially gone into the 

 t?sting of the truth rf the above statement; but so far as concerns the two bifurcation 

 planes at the ends of any one axis, my experience makes me hesitate to lay it down as 

 a general rule that tliey are relatively vertically oriented to each other, for the angle 

 referred to seemed to me to be much too variable and indefinite, as observed in a large 

 number of cases in various Hexactinellid species. 



