210 GEODINELLA ROBUSTA. 



DIMENSIONS OF ISOACTINE RHABDS OF VAR. CAROLAE FROM CHARLOTTE 



SOUND. 



Length 



Thickness in 



the middle 



0.37 mm. 



48 ti 



0.43 " 



47 jj. ■ 



1.2 " 



47 /< 



1.8 " 



45 /( 



2.0 " 



50 /( 



2 2 " 



52 // 



There is, however, a correlation between the length and the shape of these 

 spicules, the long rhabds being spindle shaped, the short ones cylindrical and 

 usually thickened at the ends, amphityle in character. The terminal thickenings 

 of these amphityles are more or less spherical. One of the short amphityles is 

 represented on Plate 4, fig. 16. This spicule is 0.4 mm. long and 65 /i thick in 

 the middle. Its terminal thickenings measure 80 /t in diameter. In other 

 amphityles this terminal thickening is still more pronounced. In one such, 0.42 

 mm. long and 56 ft thick, the terminal thickenings measured 73 p.. In these 

 shortened amphityle spicules the axial thread terminates in the centres of the 

 spherical terminal thickenings. I think that these short amphityles may have 

 been produced by some cause preventing the axial thread from attaining its full 

 length, but not proportionately reducing the vital energy of the silicoblasts 

 which deposit the silica, intended, as it were, for enclosing the (missing) terminal 

 parts of the axial thread, at the ends of the (short) axial thread present. The 

 influences regulating the production (growth) of the axial threads seem accord- 

 ingly to be distinct from, and to a certain extent independent of, those regulating 

 the action of the silicoblasts. In var. megasterra and var. megaclada the iso- 

 actine rhabds are as a rule spindle shaped, 1.6-2.8 mm., mostly 1.9-2.3 mm. 

 long, and 33-60 /i, mostly 40-55 thick. Cylindrical rhabds reduced in length 

 and thickened at the ends, like those described above from var. carolae, are 

 exceedingly rare in var. megasterra and seem to be absent altogether in var. 

 megaclada. As will be seen from the measurements of these spicules of var. 

 megasterra tabulated below, the terminal attenuation is very variable, and the 

 thickness of the ends correlated to the length and the thickness in the middle only 

 in so far as the two longest spicules in the list both have slender pointed ends. 



