osculum

noun

1. (obsolete)A small opening or orifice.
2. A small opening or orifice.

   “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.”

   “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, 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, 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.”