464 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
noticed, has published a text-book on the subject. An account is given 
of some of the most important researches on the subject published 
during the last fifteen or twenty years. It would be possible to point to 
passages which might have been otherwise expressed, and subjects which 
might with advantage have been differently treated, but as it is the first 
work of its kind in our language, we may be thankful for what it 
gives us. 
Mollusca. 
Malacological Fauna of the Red Sea.* — Dr. Jousseaume considers 
that the Red Sea has been supplied with animals from the Indian Ocean, 
of which it is merely a big bay. The wealth of Indo-Pacific forms in the 
Red Sea does not surprise him, as he always considered currents to be 
the most active cause in the distribution of Mollusca. 
Cleavage of Ovum of Crepidula fornicata.f — Mr. E. G. Conklin 
describes the eggs of Crepidula as being laid in pouches and fixed under 
the shelter of the shell ; here they are aerated by currents of water, 
which are swept into and out of the branchial chamber. The author 
corrects his earlier account of the relation of the axes of the embryo ; he 
finds that the “ cross furrow ” always bends to the right when the first 
furrow is in the line of vision, and to the left when the second furrow is 
in that position. It was found that the first furrow is transverse to the 
long axis of the embryo, and divides the egg into an anterior and a pos- 
terior half, while the second furrow lies in the long axis, and divides the 
egg into right and left halves. Each of the four macromeres formed by 
the first two cleavages contains the elements of both ectoderm and endo- 
derm, but only the left posterior macromere contains mesoderm. Four 
characteristically arranged ectoderm cells occupy the centre of the ecto- 
dermic area, and when thirty-six ectoderm cells have been formed these 
four central cells form the centre of a cross of ectoderm cells. In further 
development all the arms of the cross lengthen, and all save the posterior 
divide longitudinally into two parallel rows of cells. An elongated 
blastopore is formed on the ventral side, it closes posteriorly more 
rapidly than anteriorly, but at last it leaves only a small depression, 
where the mouth is finally formed. 
y. Gastropoda. 
Movements of Lymnaeus on the Surface of Water. J — Grafin M. 
von Linden has studied these familiar movements. It is not the case 
that the concave foot forms a boat which supports the snail, for the foot 
may be quite flat while the animal moves rapidly. Moreover the snail 
may swim a few millimetres below the surface. The fact is that the 
animal is able to alter its specific gravity as it pleases. On one gliding 
along the surface an air-bubble may be seen at the respiratory aperture ; 
as this bubble recedes or protrudes, the snail sinks or rises. If the 
bubble be removed or lost, the snail sinks, and before it can swim again 
it must reach the surface by creeping. The actual movement is due to 
undulatory contractions of the sole. It is especially for the sake of food 
* Arm. Sci. Nat., xii. (1892) pp. 343-63. 
f Zool. Anzeig., xv. (1892) pp. 185-8 (5 figs.). 
X Biol. Centralbl., xi. (1891) pp. 763-6. 
