On April 16, 2010, ten days after fertilization, the experimental embryos have proceeded to the pluteus stage. The larvae appear to develop normally, if slow. The spicules are shaped like a chair (viewed from the side), as they are supposed to be at this stage. This larva also shows a small hydrocoel (a coelomic sack) which will later develop into the water-vascular system , characteristic of echinoderms. The hydrocoel is connected to the outside via a hydropore canal visible as a small strand of tissue reaching to the surface of the larva in the upper left quarter of the picture. Also, if you look closely, you will see some of the cilia at the tips of the larval arms which surround the larval mouth (to the left in this picture). These cilia make up the circumoral ciliated band, used in feeding.
This is another picture of the same half-size pluteus larva as above, just at a different focal plane. Unlike some larvae, plutei (plural of pluteus) are planktotrhopic, meaning that they feed on plankton. Therefore, if a larva is to be considered normal it must be able to feed. In this picture (side view) you can clearly see the complete tripartite gut which consists of the esophagus (on the left), the stomach (a clear shape roughly in the center of the larva), and the intestine (an oval shape under the stomach). Note some green particles in the stomach. These are cells of the unicellular green alga Dunaliella tercioleta that I have been feeding to the larvae. See also my next post.