Orthonectida's a small phylum of poorly-known parasites of marine invertebrates that're among the simplest of multi-cellular organisms. Members of this phylum are known as orthonectids. Characteristics Multinucleate plasmodia give rise to sexual males and females, which are free-swimming by means of cilia. They're composed of a single layer of ciliated outer cells surrounding a mass of sex cells. When a female's inseminated, the eggs develop into ciliated larvae that seek out new hosts and then… (More on Orthonectida)