Light micrograph of Rhabditis marina, a common marine nematode (roundworm) which appears as thousands of tiny (less than 1mm) white threads in mud and among seaweeds. Anterior view showing pharynx, intestine, etc. The nematodes are among the most abundant of all animals, both in sheer numbers and in species (there are over 100,000) occuring worldwide in marine and freshwater muds and soils, and as plant and animal parasites. Like higher animals, they have a gut with mouth and anus, muscles, nerves, excretory and reproductive organs (separate sexes). They lack a circulatory system. Parasites important to man include Trichinella, Enterobius ("worms" in children), Ascaris (hookworm), Guinea worm, filaria worms and eelworms. Magnification x5 at 6x7cm.