134 KEFERSTEIN, ON SAGITTA. 
The paper concludes with some observations upon -the 
phenomena afforded by diatom-valves under polarized light. 
H. v. Mohl has stated* that he observed, by means of his 
improved polarizing apparatus, phenomena of double refrac- 
tion in the shells of certain diatoms, as, for instance, in P. 
angulatum, and his observations have since been confirmed 
by Valentin.t The double refraction, according to v. Mohl, 
is so powerful that under a Nicol’s prism even the hexagonal 
points on the surface may be seen. It was these observations 
that suggested to the author that the appearances presented 
in the Pleurosigma valve might be due to siliceous crystals. 
As this idea, as has been shown, is not maintainable, to 
what cause is the phenomena of double refraction to be 
assigned? There is no doubt of the correctness of H. vy. 
Mohl’s account as applied to dry valves, but when they are 
mounted in Canada balsam or other highly refracting 
medium the appearance is at once destroyed. There can 
be no doubt, therefore, that the phenomenon is due, not to 
double refraction, but to what the author terms depolarization 
by refraction, as is exhibited in one of Nobert’s tests when 
seen through ground glass or through fine, crossed lines. 
The very faint appearance witnessed, more especially in the 
thicker. diatoms, quite at the edge of the valv e, even when 
mounted in balsam, the author explains on the supposition 
that their structure may be laminated, and consequently that 
the substance at different depths is in a condition of varying 
tension. 
On Sacitta. By Prorrssor Krrerstern. 
Prorrssor Krrerstein, of Gottingen, in his recently pub- 
lished ‘ Investigations on the Lower Marine Animals,’{ has 
a short communication entitled ‘‘ Some Remarks on Sagitta,” 
of which the following is a translation. 
Ovary.—In specimens of a Sagitta about 9 mm. in length, 
which was taken pretty frequently at St. Vaast, and which 
agrees for the most part with that so admirably described by 
R. Wilms, and denominated by Joh. Miiller S. setosa, I found 
* «Botanische Zeitung,’ 1858, p. 10. Poggendorfl’s ‘Ann.,’ 1859, 
Bd. 108, pp. 179—185. 
pas Die Untersuchung der Pflanzen-oder der Thiergewebe in polarisirtem 
Lichte,’ 1861, p. 203. 
t Forming "Part 1 of vol. xii of Siebold and Killiker’s ‘Zeitschrift fiir 
wissenschaftliche Zoologie,’ 1862. 
