Ecdysoan Protostomes  - The second major clade of Protostomes that we will examine are members of Ecdysoa.  This group is supported by molecular genetics and characterized by undergoing ecdysis (molting).  Ecdysoans have a hard, extra-cellular covering which is secreted by cells of the epidermis and provides the animal protection.  As the animal grows, it must shed the old covering (called a cuticle or shell) and then produce a new one.  We will examine Phylum Nematoda and Phylum Arthropoda as examples of Ecdysoans.

Phylum: Nematoda- [thread worms] roundworms. (Hickman et al. chapter 15)

Nematodes are pseudocoelomates and as such, have a body cavity (called the pseudocoel) is lined with both endodermis and mesodermis.  The complete digestive tract runs through the pseudocoel from the mouth on the head to the anus on the animals posterior end. Complete digestive systems have the advantage that the individual can constantly feed and sections can be specialized for processing food in different ways. Although nematodes have a complete digestive system, there is little specialization.

Nematode internal anatomy

Nematodes have an acellular covering called the cuticle. The cuticle provides protection for the animal, reduces water loss, and is a firm covering to withstand the hydrostatic pressure of the pseudocoelom. Nematodes have a hydrostatic skeleton and move by contracting muscles on alternating sides of the body to wiggle through the environment. Nematodes are the first group we have examined that had been relatively successful in adapting to terrestrial habitats. Although many species are parasitic on plants and animals, nematodes are extremely abundant in soil. While the cuticle reduces water loss, It also stops waste products such as CO₂ and ammonia from diffusing out of the body. The excretory system is composed of two lateral lines that run longitudinally down either side of the body and removes wastes and maintains water balance. The nervous system of the nematode is composed of a dorsal and ventral never cord.

Nematode cross section

Nematodes are dioecious and exhibit sexual dimorphism; which means that males and females are morphologically different. Females are slightly larger and males have a "hooked" posterior end and a pair of copulatory spicules used in aligning with the female during copulation.

Hookworm life cycle

The second most diverse group of animals, it is estimated there are over 500,00 different species of nematodes. They live in all possible moist and aquatic habitats and represent the most abundant group of animals known (they can number in the billions/ acre of topsoil). Ten species are dangerous to humans as parasites, 50 others parasitize humans with minimum impact.

Ascaris lumbricoides - infect the intestines of humans.
Trichinella spiralis - pigs and humans. Infect muscles.
Necator americanus - hookworm, infect skin from soil.
Pinworms - most common of all human roundworms.

Phylum: Arthropoda [jointed foot] insects, crustaceans, spiders and ticks, millipedes, centipedes (hickman chapters 18,19,20).

Arthropods are the most successful group of animals on earth. There are over 900,000 described species and approximately another million species yet undescribed. Arthropods are found in all habitats and are often the most abundant organisms. Next week will explore Arthropod diversity and examine some of the adaptations that have made them so successful

The arthropod body plan is segmented (metamerism) and often is composed of three sections (head, thorax, and abdomen) of fused segments (tagmosis). Tagmosis is the process by which segments are fused to form larger body sections. Support for the animal comes from a hardened exoskeleton of chitin, a polysaccharide. Because the exoskeleton can not expand as the animal grows, it is periodically shed by a process called molting (also called ecdysis). After the animal molts, the individual produces a new, larger exoskeleton. Because the cuticle is hardened over most of the body, movement of body parts only occurs at joints where the cuticle is thin and flexible.

Crayfish external anatomy

Respiration in arthropods varies between groups but is usually by gills in aquatic habitats and a tracheal system in terrestrial forms. The tracheal system is composed of tubes that distribute air throughout the body. Some spiders have a specialized lung, called a book lung. Although most arthropods have specialize respiratory structures, diffusion of gases across thin areas of the cuticle also contributes to respiration. The circulatory system is open.

Crayfish cross-section

Nitrogenous waste of terrestrial forms is usually converted to uric acid to reduce water loss. Malpigian tubules excrete uric acid into the digestive system where it is removed from the body with the feces. In aquatic forms, nitrogenous waste is removed as ammonia. Specialized structures, called antennal glands, in crayfish funtion in excretion and help maintain proper water balance in the body.

Crayfish internal anatomy

The sexes are usually separtate (dioecious) in Arthropods and many have specialized life stages within the life cycle that minimize competition between young and old individuals of the same species. The body of larvae is often very different from that of the adult (compare a catepillar to a butterfly) and go through a pupal stage where they metamorphose into the adult form.

Grasshopper dissection

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