The ultrastructure of the mature spermatozoa of three polychaetes is described: the amphinomid Eurythoe complanata, and the maldanids Micromaldane sp. and Clymenella sp. Spermatozoa of polychaetes and other Metazoa utilizing external fertilization conform to a basic structural plan presumably adapted morphologically and physiologically for aquatic swimming in possessing, in 'anterior-posterior' sequence, a cap-like acrosome, a subspheroidal nucleus, a midpiece composed of unmodified rounded mitochondria, surrounding two centrioles, and a free flagellum. These, previously called primitive spermatozoa, are renamed ect-aquasperm, a functional term placing no phylogenetic significance on this form of fertilization. The ect-aquasperm is well exemplified by Eurythoe complanata. Other sperm types (including those previously called modified or aberrant sperm) are re-classified into two groups on a functional basis. Ent-aquasperm are again released freely into the ambient water but differ from ect-aquasperm in being gathered by, or in some other way reaching, the female. This is often coincident with lecithotrophic development which is commonly reflected by elongation of the sperm nucleus. Putative ent-aquasperm are exemplified here by Clymenella, with minimal modification, slight elongation of the nucleus, and by Micromaldane which shows profound modification notably in elongation of the mitochondria pairwise along the greatly elongated nucleus and development of a specialized centriolar-nuclear bridge complex in a deep implantation fossa of the nucleus. The final major sperm type, the introsperm, has no contact with water when passed from male to female and is universal for terrestrial Metazoa. Modes of introsperm transfer are discussed.