NELSON, a seaport of New Zealand, the seat of a bishop and capital of a provincial district of the same name; at the head of Tasman Bay on the northern coast of the South Island. Pop. (1931) i o,800. The woods and fields in the neighbourhood abound with English song-birds, and the streams are stocked with trout; while the suburbs and the district surrounding are famous for apple orchards; and hops are extensively cultivated. The town possesses boys' and girls' colleges of high repute. The cathedral (Christ Church) is finely placed on a mound which was originally intended as a place of refuge from hostile natives. The harbour, with extensive wharves, is protected by the long and remarkable Boulder Bank, whose southern portion forms the natural break water to that anchorage. The settlement was planted by the New cavity lined with cuticle or directly into the oesophagus, which is usually a three-sided muscular organ, but occasionally consists of a single column of cells surrounding a cuticular tube. The intestine is usually straight, and generally opens through a short rectum and an anus near the hinder end of the body.
The nervous system consists of a perioesophageal ring giving off nerves anteriorly and posteriorly. The chief sense organs are tactile papillae or bristles on the cuticle, situated mainly near the extremities. In certain free-living forms ocelli or "eye-spots" are present internally in connection with the oesophagus, and sometimes possess lenses. A pair of special lateral cephalic or gans, or "amphids," of uncertain function, exists in most free living forms.
Nematodes may be oviparous, ovoviviparous or viviparous. The free-living forms commonly produce relatively few and large eggs or young. Most of the parasitic forms, however, produce eggs or young, of much smaller relative size, in enormous num bers. These generally reach the exterior with the faeces of the host. The life-history of parasitic species may involve only one
host, in which case the larvae (sometimes still in the egg-shell), may be taken in passively by the mouth with contaminated food or water, or may actively penetrate the skin or mucous mem branes. On the other hand, an intermediate host (usually an arthropod or lower vertebrate) may be required, in whose tissues or body-cavity the third-stage larva remains encapsuled until swallowed, along with its host, by the final or definitive host.
The sexes are usually distinct. In the male the genital canal and the intestine open into a common cloaca. In the female the anus and genital aperture (vulva) are separate, the latter opening on the ventral surface at a very variable distance in front of the former. The males are usually smaller and less numerous than the females. The reproductive glands are blind, tubular and often much-coiled organs, continuous with their ducts. In the male there is most often a single testis (occasionally two). The spermatozoa are usually tailless, rounded or amoeboid cells. Secondary male organs in the form of chitinoid copulatory spicules and one or more "accessory pieces" are usually pres ent in connection with the cloaca. The vagina of the female leads into a uterus with usually two (sometimes more, or only one) long branches, acting as egg-reser voirs, and each connected distally with its own ovary. The ova are fertilized in the uterus, and here also, in oviparous species, a firm chitinoid shell is usually formed round them before oviposition.