We are raising bipinnaria larvae of the sea star, Pisaster ochraceus in our Embryology class at OIMB! I was particularly interested in the development of coelomic sacs in the bipinnaria. These pictures show three different stages of development of coelomic sacks with larval anterior oriented up.
The top picture is of a 2-day old late gastrula, and the elongated cylinder inside the gastrula is the archenteron, or primary gut. You may notice that the tip of the archenteron is slightly T-shaped. This is because during gastrulation, before the archenteron makes contact with the oral epithelium, two coeloms form as pouches from the tip of the archenteron, during a process called enterocoely.
The coeloms bud off from the archenteron forming a sac on the left and right side of the gut as you can see in 4-day old bipinnaria larva in the middle picture.
The bottom picture is a dorsal view of a 12-day old bipinnaria with well-developed coelomic sacs. It is interesting to note the larger size of the left coelom because the two coeloms have different roles in development. The left coelom, called the hydrocoel, is connected to the dorsal epithelium via a hydropore canal, and opens to the environment via a small round hole, called the hydropore, which you can see as a small dark shape to the upper left of the larval stomach (the upside-down pear-shaped structure occupying the posterior portion of the larva). The hydrocoel will form the water-vascular system of the adult sea star. I look forward to continuing to watch the development of the Pisaster ochraceus larvae as well as the development of their coelomic sacs!