The Mind of 
	a Worm

GENERAL DESCRIPTION OF C. elegans

C. elegans is a small, free-living, soil nematode worm. It has a generation time of about 3.5 d and grows to a length of 1.3 mm and a diameter of 80 um if there is a plentiful supply of food. It is a self-fertilizing hermaphrodite, one animal generally giving rise to about 300 offspring. Occasionally, at a frequency of about 1 in 103, a male is produced, which is capable of mating with the hermaphrodites. C. elegans can easily be cultured in the laboratory on bacterial lawns grown on an agar substrate. Mutations may be readily produced by a variety of mutagens and will segregate out as homozygous clones without having to set up back-crosses. These characteristics make the animal very amenable to genetic analyses, and many behavioural and morphological mutants have been mapped (Brenner 1974; Swanson et al. 1984).

Behaviour

The animals pass through four larval stages before reaching adulthood: Ll, L2, L3 and L4. Each stage is terminated by a moult. If food is scarce, animals can go through an alternative developmental sequence in which a resistant 'dauer' larval form is produced at the L2 to L3 moult. Dauers can survive extreme conditions (desiccation and lack of food) for long periods until conditions improve and food becomes available, at which time they will moult and become normal adults (Cassada & Russell 1975; Riddle et al. 1981). Several structural changes occur on entering the dauer stage, including alterations to the endings of some sensory receptors (Albert & Riddle 1983).

C. elegans normally inhabits the interstices between damp soil particles or in rotting vegetation. It lives in a film of water and is held to solid surfaces by surface tension. Locomotion is achieved by dorso-ventral flexures of the body, which give rise to sinusoidal wave propagation along the length of the body. This can either be in the anterior-to-posterior direction, giving rise to forward motion, or in the posterior-to-anterior direction, giving backward motion. The head has an extra degree of freedom, in that it can make lateral as well as dorso-ventral movements. The dorso-ventral flexures (with the consequential sinusoidal posture of the body), combined with the surface tension forces, constrain the animals to lie on their sides. The L1, dauer and adult stages have longitudinal lateral ridges of cuticle, the alae, which may act to increase lateral friction and minimize sideslip. The thickness of the water film is quite critical; too thin or no water film results in the animals' becoming desiccated and dying, whereas if the film is greater than their diameter they are not held down to the surface and are unable to make any progress. C. elegans can move well on an agar surface even though this must be quite different from its normal habitat. If there is no food available locally it will move forward for quite long periods with occasional short intermissions of reversing. When it locates food it starts eating and stops moving, except for short foraging excursions forwards and backwards. Eggs tend to be laid only when the hermaphrodites have a plentiful food supply.

C. elegans responds in a regulated manner to a number of sensory stimuli: it will chemotax up a gradient of chemical attractant or down a gradient of repellant (Ward 1973; Dusenbery 1974); it will avoid regions of high osmolarity (Culotti & Russell 1978); it will actively maintain itself at an optimum temperature in a temperature gradient (Hedgecock & Russell 1975) and it will respond to light touch by moving away from the point of stimulation (Chalfie & Sulston 1981). In addition to these responses, the worm presumably uses its mechanosensory system to navigate through the interstices between soil particles in its natural habitat. Mating-specific behaviour is exhibited only by the male (Hodgkin 1983), which has additional neural circuitry in the tail for controlling copulation (Sulston et al. 1980).

Structure

The animal is ensheathed in a tough impermeable elastic cuticle, which is laid down by a system of underlying hypodermal cells. The body cavity (the pseudocoelome) is maintained at a high hydrostatic pressure relative to the outside; it is this pressure, acting on the elastic cuticle, which gives the animal its rigidity (the so-called hydrostatic skeleton (Crofton 1966).

There are four longitudinal ridges running down the inside of the body cavity: two medial and two lateral. These ridges consist of a ridge of hypodermis adjacent to a bundle of nerve processes, the whole structure being bounded by a basal lamina. Body movements are mediated by four strips of muscle cells running in four quadrants between these longitudinal ridges. Muscle cells have no obvious attachment points at either end and probably have attachments to the hypodermis distributed along their length. They act to deform the cuticle elastically against the stress produced by the turgor pressure.

Food is pumped into the animal and processed by a prominent pharynx. This is a virtually self-contained organ with its own musculature, epithelium and nervous system, and has been described in detail by Albertson & Thomson (1976). The pharynx probably functions as a largely autonomous unit, although there are two interneurons that originate in the central nervous system and enter it. These interneurons (RIP) are exclusively postsynaptic outside the pharnyx and so probably mediate the overall control of pharyngeal pumping from the central nervous system. The pharynx is used for ingesting food (usually bacteria), concentrating it by filtration and then grinding it, and probably also for secreting digestive enzymes from its gland cells (Albertson & Thomson 1976). The processed food is pumped into the intestine, which has a lumen lined with microvilli. The intestine is connected with the anus; defecation is controlled by three sets of specialized muscles (figure 12).

There is an excretory system, which consists of a single excretory canal cell arranged in an, 'H' configuration (Bird 1971).The two arms of the H run longitudinally down the lateral lines. These are joined by a cross bridge, which is connected to the excretory duct on the ventral side; this opens to the outside of the animal via the excretory pore situated on the ventral mid-line. Two ventrally situated 'gland' cells have anteriorly directed processes, which fuse and connect to the lumen of the excretory canal near the pore (Nelson et al. 1983). These processes continue running anteriorly on the ventral surface of the ventral nerve cord (figure 16) until the nerve ring is reached, where they terminate. The function of these glands is not yet known.

The adult hermaphrodite reproductive system consists of symmetrical pairs of uteri, oviducts, spermathecae and ovaries, which are joined at the uteri and connect to a vulva. This is situated on the ventral mid-line about halfway down the body (Hirsh et al. 1976). During development, sperm are produced before oocytes and are stored for subsequent use. Egg-laying is mediated by a set of sixteen muscle cells, eight of which act to squeeze the contents of the uteri and eight to open the vulval orifice (figure 11).

The male gonad joins the rectum via the vas deferens to form a cloaca in the tail (Sulston et al. 1980). The cloaca is surrounded by a large, fan-like, copulatory bursa, which is richly endowed with sensory endings. These endings are derived from male-specific neurons, which are generated post-embryonically along with other neurons in the male. The male also has extra ventral body muscles and muscles that control the copulatory spicules (Sulston et al. 1980).


Web adaptation, Thomas Boulin, for Wormatlas, 2001, 2002