I have had an opportunity during the past summer, through the kindness of my friend Captain Wethered, of observing with care some of the earlier stages in the development of Holothuria inhœrens of O. F. Miiller, so well known to microscopists from the remarkable character of its calcareous armature. This singular Holothurid is abundant, buried in mud banks, at, and a little above, low water mark on the shores of Belfast and Strangford Loughs. I had long been most anxious to trace its mode of reproduction, but every attempt, whether by sifting the surface of the water with a towing net for a swimming larva, or by examining the plants growing on the banks for an early creeping stage, signally failed. In the mean time, Captain Wethered contrived thoroughly to domesticate three or four mature Synaptæ in a small tank; and after they had been upwards of a year in confinement, he was rewarded for his care and patience by a brood of young. He was absent from home for some days in the beginning of June of the year 1861, and, on his return, he detected several extremely minute transparent bodies moving on the surface of the glass of the Synapta tank. On examining these with the microscope he found that they were young Echinoderms, and, even at this early stage, their mode of using their rudimentary tentacles was so characteristic as to leave but little doubt of their parentage. Captain Wethered kindly placed these embryos at my disposal, and I examined them carefully from time to time. There still, unfortunately, remains a doubt, whether there may not be a still earlier condition of swimming larva. The embryo, however, when first observed, was extremely minute (‘8 mm. in length); its structure was very rudimentary, and even in the simplest and most minute embryos there was not the slightest trace of larval appendages. I am, therefore, inclined to believe, that no independently organised pseudembryo is produced, but that the Echinoderm is developed at once from a simple ciliated germ. I shall not, however, consider my task completed, till this point be fairly cleared up; and, in the mean time, I merely offer the present communication as a contribution to the history of a limited period in the development of the young.