Ecdysozoans

Ecdysozoans are animals that undergo ecdysis. Ecdysis is a specific type of molting and replacing of the external protective cuticle, or exoskeleton. Organisms that undergo ecdysis include tardigrades, nematodes, arthropods, onycophorans, and a few other minor phyla. Like the lophotrochozoans, they exhibit protostome development, bilateral symmety,l and triploblasty.

Tardigrades

Tardigrades, or water bears, are microscopic animals that are famous for the ability to withstand extreme environmental conditions. They are bilaterian, triploblastic, and coelomate. They are distributed worldwide and inhabit freshwater, marine, and terrestrial environments. They are often found on moss and lichens. There are more than 1,000 described species.

Tardigrades can enter a dormant state called a tun. While in this state, their metabolism is almost completely halted. While in the tun state, tardigrades have been observed to survive freezing, complete dehydration, pressures ranging from a vacuum to 1,200 atmospheres, radiation a thousand times greater than a human can survive (5,000+ Gy), temperatures up to 150°C, and the conditions of outer space.

Nematodes

Nematodes are bilaterian, triploblastic, pseudocoelomate worms. They range from microscopically tiny to over 8 meters long (Placentonema gigantissima, a parasite of sperm whales). Nematodes have an outer covering called the cuticle, which protects them from dehydration. The cuticle is molted as the animal grows. The nervous system includes a brain that is ring-shaped and surrounds the pharynx, and a ventral nerve cord. Nematodes have a complete gut, with a mouth at the anterior end and an anus near the posterior end, but without a stomach. The muscular pharynx connects directly to the intestine.

Nematodes are often dioecious, but a few species, including the best-studied (Caenorhabditis elegans) are androdioecious, meaning there are hermaphrodites and males. Some nematodes reproduce via parthenogenesis. Nematodes inhabit almost every ecosystem on earth. Many are parasitic, and a few species are common and significant human parasites. There are about 25,000 described species.

Arthropods

Arthropods are the most numerous of animals, and account for over 80% of described animal species. They are distributed worldwide and inhabit marine, freshwater, terrestrial, and aerial ecosystems. Although each group has unique anatomical features, all arthropods share several distinguishing characteristics. All arthropods have an exoskeleton made of chitin, which is shed via ecdysis multiple times during the arthropod’s lifespan. Arthropods display segmentation of the body. Body regions of arthropods are called tagma. For instance, insects have three tagmata – the head, thorax, and abdomen. Jointed appendages are another key characteristic of all arthropods.

Arthropods have a central cavity that contains the blood, called the hemocoel, and the open circulatory system is regulated by a tubular or single-chambered heart. Respiratory systems vary depending on the group of arthropod: insects and myriapods use a series of tubes (tracheae) that branch through the body, open to the outside through openings called spiracles, and perform gas exchange directly between the cells and air in the tracheae, whereas aquatic crustaceans utilize gills, terrestrial chelicerates employ book lungs, and aquatic chelicerates use book gills.

Most arthropods reproduce sexually, although some can reproduce by parthenogenesis. A few arthropods are hermaphrodites, including some barnacles. However, most are dioecious, with distinct male and female sexes. Some display sexual dimorphism and sexual selection. Most arthropods lay eggs, but some are viviparous, giving birth to live young. Some species of scorpions and spiders exhibit parental care.

Major Groups of Arthropods:

Trilobites

The earliest trilobites appear in the fossil record at the beginning of the Cambrian Period, around 521 million years ago. In fact, trilobites are so widespread and common that they are used by stratigraphers to data Paleozoic strata.

Trilobites had a segmented body, with three tagmata – the head, thorax, and pygidium (tail piece). The thorax has many segments. Trilobite fossils are generally only of the exoskeleton, which was chitinous and reinforced with calcite. Trilobites had jointed appendages and a single pair of antennae.

By the time trilobites appear in the fossil record, however, they were already very diverse and widely distributed, suggesting that they evolved prior to the Cambrian Period. The classification of trilobites is disputed and may be impossible to resolve since no molecular data are available; the tree shown is one hypothesis, other groups place trilobites as the closest relative to chelicerates.

Crustacea

Crustaceans are a diverse group of arthropods. Most are marine, but there are also freshwater and terrestrial crustaceans. This group includes Class Malacostraca (lobsters, crabs, krill, shrimp, crayfish, amphiopods, etc.), Class Ostracoda (seed shrimp), Class Branchiopoda (fairy shrimp, clam shrimp, etc.), Class Maxillopoda (barnacles, copepods, etc.), and others.

Crustaceans have two tagmata, a cephalothorax and an abdomen. The cephalothorax is covered by a plate called the carapace. Crustaceans have a chitinous exoskeleton that is shed by molting whenever the animal increases in size. The exoskeletons of many species are also infused with calcium carbonate, which makes them even stronger than in other arthropods. Crustaceans have an open circulatory system where blood is pumped into the hemocoel by the heart, which is in the dorsal aspect of the thorax.

Myriapoda

Subphylum Myriapoda includes 13,000 described species and is comprised of arthropods with numerous legs. Despite the names centipede and millipede, these arthropods do not have 100 or 1000 feet. The number of legs may vary from 10 to 750. All myriapods are terrestrial animals and prefer a humid environment.

Myriapods are typically found in moist soils, decaying biological material, and leaf litter. Centipedes bear one pair of legs per segment, mandibles as mouthparts, and are somewhat dorsoventrally flattened. The legs in the first segment are modified to form forcipules (poison claws) that deliver poison to prey like spiders and cockroaches, as these animals are all predatory. Millipedes bear two pairs of legs per diplosegment, a feature that results from embryonic fusion of adjacent pairs of body segments, are usually rounder in cross-section, and are herbivores or detritivores. Millipedes generally have more legs as compared to centipedes. Myriapods have eyes called stemmata, which are thought to have evolved from compound eyes.

Hexapoda

The name Hexapoda denotes the presence of six legs (three pairs). Hexapods are characterized by three tagmata — head, thorax, and abdomen. The thorax bears the wings and legs. Amongst the hexapods (and arthropods, and animals), insects are by far the largest class in terms of numbers, species diversity, and biomass. There are over 1 million described species, which is certainly a gross underestimate of total insect diversity. Insects are extremely successful for several reasons. First, they were among the first animals to colonize the land. Second, they have evolved true flight, allowing them to occupy a variety of habitats, escape predators, and search for mates. Third, they coevolved with the seed plants, particularly the angiosperms.

Insects typically have one pair of sensory antennae, mandibles as mouthparts, a pair of compound eyes, and some ocelli (simple eyes) along with numerous sensory hairs. The thorax bears three pairs of legs and two pairs of wings in most insect groups, although some have lost one or both pairs of wings. The nervous system of insects is complex, including a number of ganglia and a ventral, double nerve cord. Sense organs are complex and acute. In addition to ocelli and compound eyes, some insects are quite sensitive to sounds, and their chemoreceptive abilities can be astounding. Insects are dioecious and fertilization is internal in most.

Insects undergo metamorphosis. In holometabolous insect species, newly hatched young are completely different in appearance from adults. These larval forms usually have a different morphology and ecological niche than adults. When larval growth is completed, the larva stops feeding and builds a case or cocoon around itself. While encased, the larva undergoes a complete transformation, and a fully-formed adult emerges. Hemimetabolous species undergo a more gradual process, in which the newly hatched young are more similar to the adult but are small in size, lack wings, are sexually immature, and may differ in other, relatively minor ways as well. The young in these insects are called nymphs.