Development begins with a bilaterally symmetrical embryo, with a coeloblastula developing first. Gastrulation marks the opening of the "second mouth" that places echinoderms within the deuterostomes, and the mesoderm, which will host the skeleton, migrates inwards. The secondary body cavity, the coelom, forms by the partitioning of three body cavities. The larvae are often planktonic, but in some species the eggs are retained inside the female, while in some the female broods the larvae.
Development begins with a bilaterally symmetrical embryo with a coeloblastula developing first Gastrulation marks the opening of the "second mouth" that places echinoderms within the deuterostomes and the mesoderm which will host the skeleton migrates inwards The secondary body cavity the coelom forms by the partitioning of three body cavities The larvae are often planktonic but in some species the eggs are retained inside the female while in some the female broods the larvae