Biology 3B Laboratory Invertebrates II Page 1 of 17 Biology 3B Laboratory Invertebrates II: Annelida, Nematoda, Arthropoda, Onychophora, Echinodermata Objectives • To understand the basic differences among the invertebrate animal phyla • To investigate and learn the obvious external and internal characteristics of annelids, nematodes, arthropods and echinoderms • To investigate at the microscopic level the organization and function of selected tissues and cells within these groups Figure One. Cladogram of the Major Animal Phyla based upon SSU-rRNA INTRODUCTION In this laboratory, we will continue to survey the remaining four invertebrate phyla: Annelida, Nematoda, Arthropoda and Echinodermata (figure 1). We have already studied two of the five major protostome phyla. Of the remaining three major phyla of protostomes in which we will study in this laboratory, only the arthropods and annelids exhibit metamerism, the division of the body into segments. Segmentation is advantageous during development, where greater efficiency is obtained by constructing a whole organism out of identical somites or segments. In the adult, locomotor activity is enhanced because of the independent nature of each segment and the flexibility afforded by a series of segmented parts. Segmentation also gives these phyla a survival advantage. Since many segments are similar to other segments in form and function, damage to one or several segments does not necessarily compromise body functions.Biology 3B Laboratory Invertebrates II Page 2 of 17 PHYLUM ANNELIDA Members in the phylum Annelida are often referred to as segmented worms because of their segmentation, a distinguishing characteristic that sets them apart form other animals. The most recognizable members include the earthworms (terrestrial habitat), leeches (terrestrial and freshwater), and marine worms. All annelids are triploblastic, bilaterally symmetrical, and eucoelomate. In addition, annelids exhibit a body wall with both longitudinal and circular muscle layers (which, along with segmentation mentioned above, allows these animals to be quite mobile). They have a complete digestive tract. Their nervous system shows some degree of cephalization with a “brain” and two ventral nerve cords that running the entire length of the body. They have a closed circulatory system with aortic arches that act as the “heart” to pump blood through muscular blood vessels. They also have a well developed excretory system which removes waste from the blood and coelom. There are three major classes within the phylum Annelida, described below. Class Polychaeta - mostly marine worms, such as Nereis (the clamworm) Class Hirudinea - the leeches (predominantly freshwater), such as Hirudo Class Oligochaeta - mostly freshwater and terrestrial worms, such as Lumbricus (the earthworms) OBSERVATION OF POLYCHAETA CLASS POLYCHAETA Polychaete worms are mostly a marine group of worms characterized by many segments with a pair of parapodia with numerous setae (figure 2). They have a distinct head with eyes, palps and tentacles. o Examine a clamworm (Nerius). These are the “typical” polychaete worms that can be found living in the mud and debris of shallow coastal waters. Using the dissecting scope, observe the head region and find the following: eyes, mouth on the ventral side, jaws, and tentacles. o Examine one of the segments. Locate a parapodium on one side a body segment. Parapodia function in locomotion and respiration for polychaetes. Each parapodium is comprised of two lobes which bear numerous setae (the reason for the class name). Figure 2: Structure of a clamworm (Nerius) OBSERVATION OF OLIGOCHAETA CLASS OLIGOCHAETA Like polychaete worms, oligochaete worms are also segmented both outside and inside. However, oligochaetes do not have parapodia, their head is less developed and they have fewer setae. TheBiology 3B Laboratory Invertebrates II Page 3 of 17 most noticeable external feature in this group is the clitellum (figure 3). Most members in this class are either terrestrial (most) or inhabit freshwater. o Obtain an earthworm (Lumbricus) and place the animal in a dissecting tray. You may need a dissecting scope to fully appreciate the external anatomy. The first four segments comprise the head region. Find the mouth on the first segment (figure 3). The prostomium overhangs the mouth. They have a complete digestive tract that terminates on the last segment with the anus. o The most obvious external feature is the clitellum, a swollen area in the anterior third of the specimen. This region functions in reproduction by secreting a mucous which holds the participants together during sperm exchange and cocoon formation around the fertilized eggs. o Orient the worm dorso-ventrally by locating the tiny setae (hairs). Run your fingers along the animal to feel the rough texture produced by the setae. Four of these structures are found on the ventral surface of each metamere. They provide traction during locomotion. o Starting with the segment that holds the mouth, locate segment 14. Observe the openings for the oviducts (female pore) on the ventral surface. Find the sperm ducts (male pore) on the ventral surface of segment 15. Figure 3: Structure of an earthworm. We will examine the internal structure when we begin the systems. OBSERVATION OF HIRUDINEA CLASS HIRUDINEA The best known member in this class is the freshwater leech. Other members can be found on land and the marine environments. Members in this class typically have 33-34 segments with a clitellum. Most do not have setae and no members have parapodia. Members have both anterior and posterior suckers. o Examine representative members in this class. The medicinal leech, Hirudo medicinalis, secretes an anticoagulant on the host at they parasitized them. This leech was commonly used in the practice of blood-letting. It is still used today to increase circulation to surgical areas, especially with finger reattachments. Note the smaller oral sucker and larger posterior sucker.Biology 3B Laboratory Invertebrates II Page 4 of 17 Figure 4: Comparison of body cavities.PHYLUM NEMATODA Triploblastic pseudocoelomates The development of a body cavity (coelom) is considered a major evolutionary advantage over those animals which do not possess a body cavity (acoelomate). As you have already learned,