The Pentacrinoid Stage of Antedon rosaceus

Abstract

I WAS somewhat surprised at finding this summer, in Lamlash Bay, on the east coast of Arran, Antedon rosaceus in the pentacrinoid stage readily obtainable up to the end of September, and would be glad to hear from others who have been observing Antedon, their experience of the duration of the stalked condition. It is well known that the adult Antedon rosaceus is abundant at Lamlash, and that young specimens in the pentacrinoid stage are common on Laminaria in the earlier part of the summer; but I have always found the “pentacrinoids” rare or absent during August, and I have certainly never before found one in September. I find that the late Sir Wyville Thomson states, in his memoir “On the Embryogeny of Antedon rosaceus” (Phil. Trans. 1865, p. 513), that the ova are mature towards the end of May or beginning of June, and that, although the time spent in the larval stages may be to a certain extent shortened or prolonged by surrounding conditions, the disengagement of Antedon rosaceus from its stalk “constantly occurs between the middle of August and the middle of September” (p. 517). From this one would not expect to find any specimens in the pentacrinoid stage after the middle of September. This season, however, while dredging chiefly in the southern part of the bay near King's Cross Point, I obtained young stalked Antedons nearly every day between September 15 and 25. I generally got one, two, or three specimens in a forenoon's dredging (usually four or five hauls of the dredge). On September 27, the last day I dredged, I found, on some Facus brought up from six or seven fathoms at the south entrance to the bay. upwards of twenty specimens of “pentacrinoids.” They were of all sizes, from 3 mm. up to 1 cm. in length of stalk. The last were evidently just ready to be set free, and in fact several of them became disengaged from their stalks while I was watching them in a glass dish during the afternoon. The smaller specimens obtained that day were, from their structure, evidently very much younger, and could not have become free for a considerable time: how long I do not know, and would be glad to learn. Probably they would still have been in the pentacrinoid condition had they lived.