VI. The development of the calcareous test of Echinocardium cordatum

The most important stages were obtained through the agency of Dr. Th. Mortensen, of Copenhagen, who kindly sent to Prof. E. W. MacBride, F. R. S., a tube containing a very large number of small urchins belonging to the speciesEchinocardium cordatum . These specimens, which represented all stages from metamorphosis (one specimen—an imago—still had large pieces of larval spicules attached to it) up to urchins measuring slightly over 2 mm. in length exclusive of the spines, were dredged from locality 56°33' N., 1°47' E., at a depth of 89 metres, on 12th July, 1905. Prof. MacBride handed over this material to the writer, suggesting that it might be of interest to work out the early post-larval development of the skeleton. On examining the youngest stage—the imago referred to above—it was found practically impossible to determine how many plates went to form the test, as the sutures, though present, could not be made out. Other investigators appear to have met with the same difficulty—e. g ., Théel (1892) does not appear to have analysed successfully the skeleton of the imago of the ClypeastroidEchinocyamus pusillus , although he describes the development of the permanent skeleton laid down in the larva in considerable detail.