GEODINELLA ROBUSTA. 



ISOACTINE RHABDS (AMPHIOXES) OF VAR. MEG.VSTERRA. 



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The anisoactine rhabds (amphioxes and amphistrongyles) and the true 

 styles (Plate 4, fig. 17) are sUghtly shorter and (at the stouter end) a trifle thicker 

 than the isoactines. They measure 1.1-2.3 mm. in length and 40-80 n in thick- 

 ness. In the true styles, which represent, as it were, the end of the series of 

 increasingly anisoactine rhabds, the thickness of the rounded end is generally 

 speaking in inverse proportion to the length of the spicule. Some of the styles 

 of both the specimens " var. carolae are somewhat thickened at the end and 

 appear as subtylostyle= 



Irregidar rhabds (Plate 4, figs. 6, 7, 10). Not a few of the rhabds of var. 

 carolae have a slightly undulating surface which renders their contour percep- 

 tibly wavy. Other rhabds, both of this variety and of var. megasierra, possess 

 on one side a small, rounded, well-defined protuberance 5-10 fi high, which is 

 usually nearer to one of the ends than to the centre of the spicule. Below the 

 protuberance the silica-layers forming the spicule conform to the outer surface, 

 this disturbance (upheaval) reaching right down to the axial thread and thus 

 showing that the cause of the formation of the protuberance acted before the 

 silica-layers were produced. Sometimes more than one such protuberance is 

 observed on a rhabd. In the spicule of var. megasterra (Plate 4, figs. 6-7) quite 

 a cluster of such protuberances rises from each end of the spicule. In some cases 

 the protuberance is not, as in the spicules described above, confined to one side 

 but goes nearly or quite round it, forming a more or less complete annular thicken- 

 ing. Among the irregular, blunt amphioxes of var. megasierra I have observed 

 some with an annular thickening of this kind below each of the ends. One of 

 these spicules was 1.7 mm. long and 60 jx thick in the middle. The two rounded 

 ends were respectively 30 and 36 n thick. One of the annular thickenings was 

 quite complete and situated 230 /z below the more slender end; the other was 

 not quite complete and situated 90 /z below the stouter end of the spicule. The 



