Anatomy of Animals

Roundworms

Roundworms (phylum Nematoda - meaning thread in Greek), as their name implies, are round rather than flat. They have a smooth outside body wall (called cuticle), indicating that they are nonsegmented. These worms, which are generally colorless and less than 5 cm long, occur almost anywhere and in great variety (15000 known species). There are many thousands of individual nematodes in even a single handful of garden soil. For the simple body plan and genetic sequence as well as the fast life cycle, the C. Elegans become indispensable in the biological research laboratory. Many nematodes are able to suspend their life processes completely when conditions become unfavorable; in these resistant states they can survive extreme dryness, heat, or cold, and then return to life when favorable conditions return. This is known as cryptobiosis - a feature shares

with the rotifers. Roundworms possess two anatomical features not seen in more primitive animals: a tube-within-a-tube body plan and a body cavity. The body cavity is a pseudocoelom, or a cavity incompletely lined with mesoderm. This fluid-filled pseudocoelom provides space for the development of organs, and serves as a type of skeleton. When roundworms are analyzed according to Table 01, they are seen to have features associated with advanced animals except that they are nonsegmented.

Figure 08b Roundworm Anatomy
[view large image]

Circulatory - Food digested in the gut is not distributed by any specialized vascular system. Nutrients and waste are distributed in the body cavity, whose contents are regulated by one-celled glands (in simpler species) or excretory canal along each side of the body (see Figure 08b).
Digestive - The head of a nematode has a few tiny sense organs, and a mouth opening into a muscular pharynx where food (mostly bacteria and detritus) is pulled in and crushed. This leads into a long simple gut cavity, and then to an anus near the tip of the body (see Figure 08b).
Endocrine (pseudo) - Among the many remarkable discoveries from the roundworm, Caenorhabditis elegans, is the fact that disrupting an hormonal insulin-like pathway can dramatically extend lifespan (see Genes of Ageing).

Excretory - Sponges and Cnidarians have radial symmetry. These animals possess a single digestive opening served as both mouth and anus. Even though evolution doesn't care about aesthetic, such arrangement is highly inefficient to say the least. It is commonly believed that during the transition from radial to bilateral symmetry, both openings evolved simultaneously by the lateral closure of a slit-like blastopore with the ends giving rise to a mouth and anus (Figure 08c). In deuterostomes (secondary mouth) the site of gastrulation (the site at which the endomesoderm forms) becomes the anus and the mouth is formed at a different end. The relationship is not that clear in roundworm (protostomis), and the flatworm (nemertodermatida) still retains only one opening although it has bilateral symmetry. Research in 2008 suggests another model, which hypothesizes the single opening shifts ventrally during the transition to bilateral symmetry with the anus fully formed only after protonephridia (which establishes a system of canals to expel watery waste, like the kidney in mammals) as indicated in Figure 08c.

Figure 08c Evolution of Mouth and Anus [view large image]

Immune - Roundworms have innate immunity that fights pathogens using defenses that are quickly mobilized and triggered by receptors that recognize a broad spectrum of pathogens. Lower animals such as the roundworms lack adaptive immune system, which may take days or weeks after an initial infection to have an effect.
Musculo-skeletal - Roundworms in general do not have an internal or external skeleton, but they do have a hydrostatic skeleton, a fluid-filled interior that supports muscle contraction and enhances flexibility. In addition, the epidermis of a nematode is highly unusual; it is not composed of cells like other animals, but instead is a mass of cellular material and nuclei without separate membranes. This epidermis secretes a think outer cuticle (Figure 08b) which is both tough and flexible. The cuticle is a feature shared with arthropods and other ecdysozoans. As in those other groups, the cuticle is periodically shed during the life of a nematode as it grows, usually four times before reaching the adult stage. The cuticle is the closest thing a roundwrom has to a skeleton, and in fact the worm uses its cuticle as a support and leverage point for movement. Long muscles underneath the epidermis move the body from side to side (no crawling or lifting).
Nervous and Sensory - The muscles are activated by two nerves that run the length of the nematode on both the dorsal and ventral side. Unlike other animals, where the nerves branch out to the muscle cells, a nematode's muscle cells branch toward to nerves. The ventral nerve has a series of nerve centers along its length, and both nerves connect to a nerve ring and additional nerve centers located near the head (see Figure 08b).
Reproductive - The reproductive system is complex, and many parasitic species have a very high reproductive potential. Some nematodes bear live young, the eggs matured in the female reproductive tract; but most release eggs, which develop into larvae that molt one or more times before reaching maturity. C. elegans exists either as a hermaphrodite or a male. The predominant sexual form of C. elegans is the hermaphrodite, which produces both sperm and eggs. Thus, it can self-fertilize. When it does, each animal produces about 300 progenies.
Respiratory - There is no respiratory system for the uptake or distribution of oxygen. The body cavity allows easy passage of molecules including oxygen.