Nervous System of Platyhelminthes

Platyhelminthes occupy a position of strategic phylogenetic importance in that they are the most primitive members of Bilateria and constitute the basal stock from which all higher animal phyla are thought to have evolved. The phylum Platyhelminthes is a highly diversified and versatile phylum, a fact that is reflected in the structure of the nervous system (NS), both in its gross morphology and in the cellular types and their secretory inclusions. The absence of a coelom and a proper circulatory system in flatwormsmeans that any long-distance control of processes, such as growth and development, is likely to be accomplished by a neurosecretory component of the NS. In this way, the flatworm NS may function not only as an NS per se but also as an endocrine system by releasing modulatory substances into the intercellular space close to target cells or organs, in a synaptical or non-synaptical way. Development of the NS in the early bilaterian flatworms would appear to have been...