1892.] Zoology. 863 
and natural selection. The color variation, on the contrary, has not 
been absolutely necessary, yet has nevertheless gone on in lines which, 
tending to give greater protective resemblance to the flower, have in 
the long run proved to be, perhaps, the most advantageous. I thus 
recognize three distinct lines of variation as exemplified in these Pro- 
doxidæ, and what is true of them is, I believe, true of all alliances of 
organisms. The first and most important is structural and generic ; it 
is absolutely essential and is preserved in its perfection by the 
elimination, through natural selection, of all forms departing from it. 
The second is merely coincident, not essential, but nevertheless along 
lines that are of secondary advantage. The third is purely fortuitous, 
affects superficial features in the main, is unessential (a consequence of 
the inherent tendency of all things to vary), and takes place along 
all lines and in all directions where there is no counteracting 
resistance.” 
Structure of Calcareous Sponges.—Minchin' finds in a cal- 
careous sponge, which he identifies as Leucosolenia coriacea, a peculiar 
fenestrated membrane crossing the oscular openings. This “ sieve 
membrane ” is composed of two layers of cells and crosses the tube just 
above the limits of the flagellate entoderm. Minchin, with a question, 
ee | LL alow 4 J PSR | | + wi J +} oes oe } hI} 
OE T , t 
that the membrane is formed by`the gastral cavity breaking through 
to the exterior at several points during development. Interesting 
comparisons are made with the sieve plates of several Hexactinellids. 
In a second paper’ the same author concludes from a study of Naples 
material that L. clathrus does not have the oscula permanently closed, 
but that these openings are capable of occlusion by means of a sphine- 
ter of ectoderm. Further that Haeckel’s so-called species, Ascetta 
labyrinthus, A. meandrina, A. clathrina and A. mirabilis are all differ- 
ent stages of contraction of the one species, Leucosolenia clathrus. 
On Echinorhynchus.—Two extensive works on the embryology 
of Echinorhynchus have recently appeared in Germany, one by Kaiser, 
which is not yet completed, the other by Hamann. 
The following is a brief summary of Hamann’s article. (Die 
Nemathelminthen, Jena, 1891, 119 pages, 10 plates.) 
The first stages can be studied to the best advantage on E. acus. 
The extrusion of the pole-bodies and the division into 2 and 4 cells 
1Q. J. M. S., xxxiii, 251, 1891. 
37. c., p. 490, 1892. ; . 
61 
