28.4B: Phylum Nematoda
- Describe the features of animals classified in phylum Nematoda
Phylum Nematoda
The Nematoda, similar to most other animal phyla, are triploblastic, possessing an embryonic mesoderm that is sandwiched between the ectoderm and endoderm. They are also bilaterally symmetrical: a longitudinal section will divide them into right and left sides that are symmetrical. Furthermore, the nematodes, or roundworms, possess a pseudocoelom and have both free-living and parasitic forms.
Both the nematodes and arthropods belong to the superphylum Ecdysozoa that is believed to be a clade consisting of all evolutionary descendants from one common ancestor. The name derives from the word ecdysis, which refers to the shedding, or molting, of the exoskeleton. The phyla in this group have a hard cuticle covering their bodies, which must be periodically shed and replaced for them to increase in size.
Phylum Nematoda includes more than 28,000 species with an estimated 16,000 being parasitic in nature. Nematodes are present in all habitats.
Morphology
In contrast with cnidarians, nematodes show a tubular morphology and circular cross-section. These animals are pseudocoelomates; they have a complete digestive system with a distinct mouth and anus. This is in contrast with the cnidarians where only one opening is present (an incomplete digestive system).
The cuticle of Nematodes is rich in collagen and a carbohydrate-protein polymer called chitin. It forms an external “skeleton” outside the epidermis. The cuticle also lines many of the organs internally, including the pharynx and rectum. The epidermis can be either a single layer of cells or a syncytium, which is a multinucleated cell formed from the fusion of uninucleated cells.
The overall morphology of these worms is cylindrical, while the head is radially symmetrical. A mouth opening is present at the anterior end with three or six lips. Teeth occur in some species in the form of cuticle extensions. Some nematodes may present other external modifications such as rings, head shields, or warts. Rings, however, do not reflect true internal body segmentation. The mouth leads to a muscular pharynx and intestine, which leads to a rectum and anal opening at the posterior end. In addition, the muscles of nematodes differ from those of most animals; they have a longitudinal layer only, which accounts for the whip-like motion of their movement.
Excretory System
In nematodes, specialized excretory systems are not well developed. Nitrogenous wastes may be lost by diffusion through the entire body or into the pseudocoelom (body cavity), where they are removed by specialized cells. Regulation of water and salt content of the body is achieved by renette glands, present under the pharynx in marine nematodes.
Nervous system
Most nematodes possess four longitudinal nerve cords that run along the length of the body in dorsal, ventral, and lateral positions. The ventral nerve cord is better developed than the dorsal or lateral cords. All nerve cords fuse at the anterior end, around the pharynx, to form head ganglia, or the “brain” of the worm (taking the form of a ring around the pharynx), as well as at the posterior end to form the tail ganglia. In C. elegans , the nervous system accounts for nearly one-third of the total number of cells in the animal.
Reproduction
Nematodes employ a variety of reproductive strategies that range from monoecious to dioecious to parthenogenic, depending upon the species under consideration. C. elegans is a monoecious species, having development of ova contained in a uterus as well as sperm contained in the spermatheca. The uterus has an external opening known as the vulva. The female genital pore is near the middle of the body, whereas the male’s is at the tip. Specialized structures at the tail of the male keep him in place while he deposits sperm with copulatory spicules. Fertilization is internal with embryonic development beginning very soon after fertilization. The embryo is released from the vulva during the gastrulation stage. The embryonic development stage lasts for 14 hours; development then continues through four successive larval stages with ecdysis between each stage (L1, L2, L3, and L4) ultimately leading to the development of a young male or female adult worm. Adverse environmental conditions such as overcrowding and lack of food can result in the formation of an intermediate larval stage known as the dauer larva.
Key Points
- Nematodes are in the same phylogenetic grouping as the arthropods because of the presence of an external cuticle that protects the animal and keeps it from drying out.
- There are an estimated 28,000 species of nematodes, with approximately 16,000 of them being parasitic.
- Nematodes are tubular in shape and are considered pseudocoelomates because of they do not possess a true coelom.
- Nematodes do not have a well-developed excretory system, but do have a complete digestive system.
- Nematodes possess the ability to shed their exoskeleton in order to grow, a process called ecdysis.
Key Terms
- exoskeleton : a hard outer structure that provides both structure and protection to creatures such as insects, Crustacea, and Nematoda