The settlement and metamorphosis of marine animals

[First paragraph(s)...]
Thorson (1) in discussing the choice of substratum by marine larvae, points out that further detailed knowledge of life cycles indicates that larvae possess more adaptability than has been hitherto assumed.
For instance, when plankton is collected by two-net, it is not unusual to take larvae of ophiuroids or echinoids, which are in process of metamorphosis, but in Lyttelton Harbour, completely metamorphosed ophiuroids or nearly completed metamorphosed spatangoids are taken near the bottom. Specimens taken nearer the surface are less changed or not at all.
That some larvae of benthic animals will readily metamorphose is shown by fully mature larvae of Phoronis metamorphosmg in sea-water in a glass dish by no obvious stimulus. It may be that the actinoirocha does metamorphose in open water and that the adhesive surface exposed collects detritus sufficient to weight the animal and drag it down.