IN the "Postscript" to my notes on the organization of Infusoria, dated 10th June last*, it is stated that apertures exist in the investing membrane of Spongilla, and that the particles of carmine taken in through them pass into the substance of the sponge-cells. This was added chiefly to correct an assertion made in the body of the paper, that Spongilla lived by endosmosis. I also stated that I should recur to these facts more particularly hereafter; but since then, up to within the last month, I have not had an opportunity of again pursuing the subject. I have, however, during this time, succeeded in ascertaining the ultimate structure of Spongilla, by following its development from the seed-like body, and this I will now relate. Those who are acquainted with Spongilla are aware, that it is charged towards the base with a number of seed-like bodies of a globular shape, each of which consists of a coriaceous membrane enclosing a number of delicate, transparent, spherical cells, more or less filled with ovules and granular matter, while an incrustation of gelatinous matter, charged with small spicules peculiar to the species, surrounds the exterior of the coriaceous membrane. It has also been shown that at an early period