The medusa Cosmetira pilosella Forbes is known, from the work of Miss M. J. Delap, to arise from a Cuspidella hydroid (Russell, 1936). Recently I found a small fertile colony of a Cuspidella on a dead bivalve shell dredged off Brigurd Buoy in the Firth of Clyde and this later liberated young medusæ in the laboratory. These were identified as Cosmetira pilosella, thus confirming that the hydroid of C. pilosella is a Cuspidella. There is no previous description of the hydroid or of the newly liberated medusa and so a description is given below.