Definition of "osculum"

I left them in that condition, and at 7 o'clock examined them again, when I found them still quiescent; but one of the two large groups of oscula and the new one were entirely closed, while the other osculum at the largest end of the sponge had opened to the extent of about one-third of its diameter, and the membrane presented the appearance of a series of lines or corrugations radiating from the centre to the circumference.

1857, J. S. Bowerbank, “On the Vital Powers of the Spongiadæ”, in Report of the 26th Meeting of the British Association for the Advancement of Science, John Murray, page 444

Waste water was expelled through a single osculum at about 8.5 cm per second – more than eight thousand times as fast as it circulated in the chambers and 85 times as fast as it entered the sponge in the first place.

2012, Caspar Henderson, The Book of Barely Imagined Beings, Granta Books, published 2013, page 29

Oscula are also thought to arise initially from a single porocyte (Weissenfels, 1980), but how they coordinate with other porocytes to form a larger osculum is still unclear.

2012, Sally P. Lees, April Hill, The Physiology and Molecular Biology of Sponge Tissues, Mikel A. Becerro, Maria J. Uriz, Manuel Maldonado, Xavier Turon (editors), Michael Lesser (series editor), Advances in Marine Biology 62: Advances in Sponge Science, Elsevier (Academic Press), page 30