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Saddleback BIO 3B - Invertebrates I: Porifera, Cnidaria, Platyhelminthes, Mollusca

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Biology 3B Laboratory: Invertebrates I Page 1 of 15 Biology 3B Laboratory Invertebrates I: Porifera, Cnidaria, Platyhelminthes, Mollusca Objectives • To understand the basic differences among the invertebrate animal phyla • To investigate and learn the obvious external and internal characteristics of sponges, anemones and jellies, flatworms and molluscs • To investigate at the microscopic level the organization and function of selected tissues and cells within these groups General Introduction to the Laboratory Observation of the Animals Figure One. Cladogram of the Major Animal Phyla based upon SSU-rRNA Animals originated in the oceans of the Precambrian era about 1.5 billion years ago. The first animals were multicellular, eukaryotic and heterotrophic. They were the first “predators.” By the beginning of the Cambrian period (543 mya), sponges and cnidarians were already present. During the end of the Precambrian and the beginning of the Cambrian, a huge diversification of the animals took place. This is called the Cambrian Explosion, although it spanned the Cambrian-Precambrian boundary (565 to 525 mya). Most extant phyla are directly traced to this period. During this period, animals experimented with tissue formation, body symmetry, gut tube formation, major feed structures, molting strategies and skeletal arrangements. These evolutionary experiments,Biology 3B Laboratory: Invertebrates I Page 2 of 15 through natural selection, resulted in the major animal lineages represented in Figure One. Although biologists identify approximately 35 extant phyla within the animal kingdom, we will look closely at only those nine shown in Figure One. (Warning: We will occasionally look at a few animals in other phyla! Yes, you will be responsible for those.) In this lab we will take a look at four phyla. The first two, Porifera and Cnidaria are complete natural groups; however, for logistical reasons we will split the lophotrochozoan phyla, covering only the Platyhelminthes (flat worms) and the Mollusca. The Annelida (segmented worms) will be covered in the next lab, Invertebrates II. PHYLUM PORIFERA Animals without tissues The “monophyletic origin of animals hypothesis” asserts that all animal groups evolved from the one protistan clade. They diversified into distinct branches, one of which produced the sponges (Phylum Porifera). Since no other animals appear to have evolved from the sponges, they are considered to be an evolutionary dead end. Members of this phylum are among the simplest animals. They consist of loose aggregations of cells with little or no tissue organization. There is some “division of labor” among the cells, but there are no organs. The basic body form of all sponges is a sac-like structure consisting of three layers, an outer layer of epidermal cells; an inner layer of cells, many of which are flagellated cells called choanocytes; and a middle layer of amoeboid cells that form skeletal structures of various sorts. These layers are perforated by a large number of small pores. The cavity of this sac is called the spongocoel and has at least one opening to the outside, called an osculum. The sponges are taxonomically classified based on the type of skeletal materials produced. These include calcareous spicules, siliceous spicules, or proteinaceous spongin fibers. This leads to the basic sponge taxonomy, which includes three classes. Sponges in the Class Calcarea have calcium carbonate spicules, which have three or four rays. All of these sponges are marine. The Class Hexactinellida have siliceous spicules, which are 6 rayed. These sponges are all marine and most often cylindrical in form and found in deep water. The Class Demospongiae are typically called “bath sponges” because they were used by humans for bathing. These sponges have spongin fibers, or siliceous spicules, or both. They represent over 90% of the sponges in the world, and one family is found in fresh water. Within each class, the sponges can be further differentiated by body form. In asconoid sponges the body wall is not folded; in syconoid sponges the body wall is folded into canals; and in leuconoids sponges the canals formed by the folded body wall are extensively branched. Ostia are the openings into the pores of asconoid sponges; they are the openings into the canals of syconoid and leuconoid sponges.Biology 3B Laboratory: Invertebrates I Page 3 of 15 In all sponge types, the body is designed to facilitate feeding. Water is pulled into the pores and canals by the beating of the flagella of choanocytes. The water moves into the spongocoel and is eventually forced out through the osculum. As the water passes across the choanocytes, food particles (microscopic algae, bacteria, and organic debris) adhere to the cells and are eventually taken into food vacuoles for intracellular digestion. Figure Two. Sponge body plans Observations of the Porifera The Asconoid Sponges Asconoid sponges have the simplest organization. Water enters the sponge through the ostia, drawn into the spongocoel by the beating of choanocyte flagella. Water is expelled through the single apical osculum. Look at Figure 2 and be sure you understand the function of this simple sponge. • Examine Leucosolenia, a simple asconoid sponge. Look at the whole mount and longitudinal section. Find the spongocoel, osculum, ostia, and (at higher power) choanocytes. The Syconoid Sponges example: Scypha (Class Calcarea) Syconoid sponges have a tubular design similar to the ascon sponge, but the body wall is folded. The "folds" form radial canals. Choanocytes line the radial canals rather Figure Three. Structure of a syconoid spongeBiology 3B Laboratory: Invertebrates I Page 4 of 15 than the spongocoel. • Examine a preserved specimen of Scypha (aka Grantia). Note the exterior surface of this sponge; the rough texture is due to spicules that protrude through the body wall (see Figure 3). Look closely for the small dermal ostia. You should be able to identify the basal disc, which is the point of attachment to the substrate. On the opposite end of the sponge, you will see the osculum. • Examine a prepared slide of the longitudinal section of Scypha using a compound microscope at low power. Draw the section, labeling the spongocoel; the radial canals that radiate from the spongocoel and the apopyles (the openings into the radial canals); the ostia and


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Saddleback BIO 3B - Invertebrates I: Porifera, Cnidaria, Platyhelminthes, Mollusca

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