NEMATODA Potts, 1932

Arai, Hisao P. & Smith, John W., 2016, Guide to the Parasites of Fishes of Canada Part V: Nematoda, Zootaxa 4185 (1), pp. 1-274 : 16

Phylum NEMATODA Potts, 1932 : diagnosis

Known commonly as nematodes, eelworms, threadworms or roundworms, members of the Phylum Nematoda are probably the most abundant multicellular animals on earth. Free-living species occur in terrestrial, fresh-water, brackish or marine environments. Parasitic species (facultative or obligate) live in association with plants, or with invertebrate and vertebrate animals, including humans. Life cycles of nematodes parasitic in animals or humans may be direct or involve two or more hosts. Usually unsegmented, round or ovoid in cross section, and tapered at both ends, nematodes range from less than one millimetre to over eight metres long. Female adults are usually larger than male adults. The anterior end is radially symmetrical but most organs are bilaterally symmetrical. A non-cellular cuticle, secreted by syncytial hypodermis, is shed four times during ontogeny. The hypodermis usually has four cords (chords): one dorsal, one ventral, and two lateral. Body-wall musculature one layer thick, longitudinal only. Pseudocoelom derived from embryonic blastocoel. Various external sensillae and ornamentations, including papillae, setae, amphids, phasmids, and annulations present or absent. Ganglia are associated with a circumpharyngeal nerve ring. Usually four major longitudinal nerves are present: one ventral, one dorsal, and at least one pair lateral. Mouth anterior, the anus at or near the posterior end. In some parasitic nematodes the pharynx is transformed into a glandular organ (stichosome), or mid-gut into a reserve organ (trophosome). In some nematodes the oesophagus is divided into an anterior muscular portion and a posterior glandular portion. The excretory-secretory system is glandular or tubular, a ventral pore opening near the anterior end [this system is referred to as the excretory system/pore/duct/cell throughout the rest of this part of the Guide]. Most nematodes are dioecious, some hermaphroditic or parthenogenetic. Most are oviparous, some ovoviviparous. Females usually have two ovaries, though there may be from one to six or more, and the female pore opens ventrally. The male system, usually with two testes, opens with anus into cloaca. Male tail curved or broadened into a fan-shaped bursa usually with one or two chitinized spicules; spicules sometimes absent. During copulation spicule or spicules enter the female pore, and amoeboid sperm are transferred.