sentences:  In most cases, the female's eggs float freely in the sea, but some species hold onto them with their spines, affording them a greater degree of protection.The unfertilized egg meets with the free-floating sperm released by males, and develops into a free-swimming blastula embryo in as few as 12 hours.Initially a simple ball of cells, the blastula soon transforms into a cone-shaped echinopluteus larva.