The Tardigrada inhabit terrestrial surface soil and tree bark, marine sands and sediments, and they have been reported in deep sea sediments at 5000 m (Cuénot, 1949b). They can be abundant in riparian areas, especially if the sediment is rich in primary producer protists, such as diatoms (Chromista) and chlorophyte algae (Plantae). There are about 600 species, ranging in adult size from 50 to 1000 ^m (Fig. 1.27). The general body plan differs from that of the previous groups in the following ways. The cuticle of tardigrades is reinforced in some species with plates, which can be ornamented. There are four pairs of extensions used in locomotion, the podia. Claws at the end of the podia are useful in species identification. The musculature consists of smooth muscles oriented in longitudinal, dorsal, ventral and diagonal bundles. Attachments of the end of the bundles to the cuticle permit more complicated movement. There are sensory papillae covering the head, dorsal surface and podia. The nervous system of the segments is coordinated centrally and through a brain. Pigmented cells are located on the head and form two simple eyes. The pseudocoel contains a serous solution with free cells containing storage granules and vacuoles. It participates in gas exchange, distribution of absorbed nutrients, nitrogen waste elimination and osmoregulation. Terrestrial tardigrades are sensitive to oxygen deprivation and, despite unusual resistance to desiccation and cold temperature, will die without sufficient oxygen. The
stoma has lips which facilitate suction created by the pharynx musculature. The cuticle of the pharynx is reinforced with six denticles that participate in 'chewing'. They are called macroplacoids and are made of chitin and calcium. Individuals search for organic matter and cells that can be sucked in. A stylet on each side of the oral cavity participates in ingestion. It is extensible in some species and used to pierce living cells of plants, mosses, lichens, nematodes, rotifers, other tardigrades and similar sized organisms in the habitat. Some such as Milsenium species are primarily predators. Copulation is unusual in that one or more males deposit spermatozoa through openings of the old moulting cuticle, or through the vaginal pore. After a long delay, 1-30 eggs are deposited in packets, often inside shed cuticles or debris. Eggs of some species bear protective scales. Juveniles emerge as miniature adults. As in previous taxa, some species seem to lack males and reproduce by parthenogenesis, whereas in others there is sexual dimorphism, with smaller males.
The onychophorans are similar to tardigrades (Fig. 1.28), but have more segments and a more complicated behaviour (Cuenot, 1949a). They also exhibit sexual dimorphism, with the larger females having more segments. Females lay up to one egg daily, but development of the juveniles to sexually mature adults lasts several months. Some species are known to care for the juveniles during this period. Only about 75 species are known. Their geographic distribution seems to be limited to the south of the Tropic of Cancer. They are found at high elevations as well as in lowlands throughout that region. Species vary in length from a few millimetres to 15 cm, and consist of from 14 to >40 segments, each with a pair of podia. However, individuals can become very thin and squeeze through narrow gaps and explore the top soil. Unlike the previous taxa, onychophorans require dark and humid habitats, and anhydrobiosis is not known. Their food preferences vary, with species that follow invertebrate tunnels to feed on the excreta or the invertebrate, and others which ingest rotting wood. The predators on small
invertebrates in the meso- and macrofauna are effective and more common. The prey are captured in a glue that is ejected from oral tentacles. The semi-liquid glue in larger species is ejected 30-50 cm and solidifies instantly on to the prey. They seem to be relatively indiscriminate, as laboratory cultures have been known to be cannibalistic when hungry. The stoma is bordered by a pair of claws and several labia. The oral cavity has a 'tongue' organ, with denticles and mandibles on each side of it for chewing. The pharynx and stoma are very muscular and provide the suction to ingest chewed tissue into the intestinal canal. The nervous tissue forms a brain, with a central nervous system that coordinates the segments. There are sensory papillae over the dorsal surface, as well as exterior of the podia, and a pair of antennae on the cephalic segment. A pair of eyes are used primarily to avoid light. The musculature consists of smooth muscles only, organized in several bundles. The role of the pseudocoel, or central cavity, in diffusion of gases and nutrients, nitrogen excretion and osmoregulation is facilitated by the following tissues. There is a urinary system that consists of nephridia, used in osmoregu-lation and excretion of uric acid. A dorsal tube functions as a blood vessel and heart to circulate the blood. Open at both ends, each contraction expels blood through both ends into the pseudocoel cavity. During relaxation, ostiole valves along the heart open and let blood flow back into the tube. The blood consists of a serous solution with nutrients, which facilitates gas exchange and has a cellular component. The cells are small corpuscles and active larger phagocytes. An elaborate tracheal system with openings in each segment supplements gas exchange and osmoregulation through the cuticle.
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