The Class Trematoda contains about 8,000 species of leaf-like animals called the (digenetic) flukes. The adults are endoparasites on vertebrates but many invertebrates serve as intermediate hosts, and many species of medical and economic importance!  Development is trematodes indirect; not only adults but larvae reproduce and all species have at least two hosts, one for transmission and the other for reproduction. Like monogenetic flukes, the outer covering is a tegument which does not bear cilia in the adult. Since the distal cytoplasm of the cells making up the tegument is continuous, with no intervening cell membranes, the tegument is syncytial (i.e. a mass of protoplasm containing many nuclei but not divided into cells).  The outer most zone of the tegument consists of an organic layer of proteins and carbohydrates called the glycocalyx,  which aids in the transport of nutrients, wastes and gases across the body wall, while protecting the fluke against enzymes and the host's immune system. Also found in this zone are microvilli that increase the surface area for nutrient  absorption. Movement  in flukes is entirely muscular (no cilia on the outer body wall of adults). The vast majority of flukes possess two large suckers  that are used for attachment, an anterior one called an oral sucker, which surround the mouth and a posterior one called a ventral sucker, or acetabulum.