Advanced Development of Some Echinoid Plutei.

Abstract

Employing a modification of the method of Allen and Nelson 1 for rearing echinoderm larvae, I have been able to observe the development of three Pacific coast forms. These were the sea urchins, Strongylocentrotus purpuratus and S. franciscanus, which developed to advanced pluteus stage, and the sand urchin, Dendraster eccentricus, which readily underwent metamorphosis.

About 20 early plutei were put into a sterilized finger bowl containing 75 cc. of sterilized San Miquel sea water, to which a drop of the culture of Nitschia closterium was added. The dish was then covered with a glass plate and set in a shaded place, where the temperature during the entire summer did not vary beyond the limits 15° to 17° C. Every 5 or 6 days the larvae had to be transferred to a fresh dish of modified sea water, in order to prevent their being overwhelmed with the growth of diatoms.

After 3 to 4 weeks the plutei of S. purpuratus and S. franciscanus developed a third pair of arms, into each of which a supporting skeletal spicule grew. All the arms became very long, and then deteriorated, without the animals giving indication of any steps toward metamorphosis. These larvae always rested on the bottom.

With the larvae of Dendraster eccentricus development was much more rapid and vigorous. Segmentation was approximately twice as rapid as in the sea urchins. In 48 hours the 4 armed plutei were formed in 10 to 12 days, and in 10 days the third pair of arms appeared. This last seemed to be a critical step because if the third pair of arms did not appear within two weeks, the larva remained juvenile. Almost immediately after the appearance of the third pair of arms, the fourth pair began to tlevelop from the oral plate, and were complete within the following week. Subsequently, growth in size and development of the echinus element were striking phenomena.