Turbellarian Platyhelminths

Abstract

In general terms, the epidermis of turbellarians is conceptually the simplest of any metazoan group; it is a monolayer of epithelial cells overlying muscles, uncomplicated by the presence of either a cuticle or of myoepithelial cells such as occur in some other lower invertebrates. It is, generally, a ciliated epidermis, and this ciliation provides the means of locomotion for the group, namely ciliary gliding. Even with such a simple basic structure there is considerable variety in the form this epidermis takes in different species. Some turbellarians, for example, have developed insunk epithelial cells, in which the nucleus-bearing portion of the cell lies among or below muscles of the body wall, leaving a thin surface portion to cover the muscles. Another marked variation is the development of a syncytial epidermis. More subtle variations relate to differences in the elaboration of intracellular cytoskeletal structures, of extracellular matrices such as basement membrane, and to differences in the secretory products of the epidermal cells, in particular small concretions and vesicles known as ultrarhabdites. The most dramatic differences in overall appearance of the turbellarian epidermis are caused by differences in abundance and distribution of non-epithelial elements, particularly gland cells such as rhabdite and mucous cells and sensory structures, the necks and dendrites of which protrude to the external milieu through the epidermis.