Lab exercise 2: Basal Animal Lineages: Porifera / Cnidaria General Zoology Laborarory . Matt Nelson phylum Porifera SYMMETRY The flatworms represent some of the simplest animals that possess bilateral symmetry. The term bilateral symmetry refers to a shape that has only one axis that divides the body into two halves that are mirror images of one another. Organisms such as cnidarians have radial symmetry, i.e. there are several planes that divide the body into halves that are mirror images of one another. These two types of symmetry, radial and bilateral, are the most common types found in the Animalia. However, there are more specific terms used to describe certain types of radial symmetry (such as pentaradial symmetry) and we will discuss these throughout the semester as they become relevant. Most of the sponges possess no body symmetry, and are considered to be asymmetrical, although there are some sponges that border on radial symmetry. However, this apparent radial symmetry generally does not apply to the pore system and the interior regions of the body. In the future, when we discuss body symmetry, it will be apparent that sometimes we are referring to the entire body, and sometimes only the external features of the body. In humans, for example, the outside of the body generally possesses bilateral symmetry, but the internal organs are in some cases asymmetrical. Organization Sponges are among the simplest of animals in terms of organization of the body. While they do possess differentiated cells, they do not possess true tissues, or organs, or organ systems. In many ways, the body of a sponge is not that different from a colonial group of cells. The general body structure is simple. There is an outer layer of cells that surround the body, and a layer of cells that lines the interior surfaces of the body. Sandwiched between these two layers is a gelatinous layer called the mesohyl which may contain skeletal elements (spicules), spongin fibers which act as a flexible skeleton, and amoeboid cells which move about the mesohyl carrying out various functions. Although there are specialized cells in each layer, none of these layers is considered to be a true tissue. The functionally different cell types found in sponges are the key factor in their classification as multicellular organisms. Sponges possess three major cell types: pinacocytes - These flattened cells cover the outer body of the sponge. In some sponges these cells are contractile and may possess the ability to alter the general shape of the sponge. In the simplest sponges, modified pinacocytes called “porocytes” form the pores that allow water to enter the body. Contraction of porocytes changes the size of the pore, limiting the movement of water through the sponge. amoebocytes (archaeocytes) - These amoeboid cells migrate throughout the mesohyl and are responsible for carrying out intracellular digestion and transporting the products of digestion throughout the body. Amoebocytes may also be specialized for other jobs, such as the production 1 General Zoology Laboratory. Matthew K Nelson (2011) of spongin fibers or spicules. Like stem cells, they are able to develop into any of the other specialized cell types found in the sponge. As such, they are important in the reproduction of some freshwater sponges, which will be discussed later in this exercise. choanocytes (shown here) - These are flagellated cells similar in form to choanoflagellate algae. They possess a collar which consists of a ring or microvilli (tiny cytoplasmic extensions) that surround the flagellum and are bound together by plasma membrane. The term “choanocyte” means “collar-cell.” Choanocytes allow sponges to feed. These may line one or more internal chambers in the sponge. The flagella of choanocytes beat back and forth creating the flow of water which brings in tiny food particles which stick to the collar and are ingested through phagocytosis. Most of the diversity in sponges involves the relative complexity of the canal system which carries water through the sponge for suspension feeding. The simplest sponges possess a central chamber called the spongocoel which is lined with choanocytes. Water comes into the spongocoel through pores (ostia) in the body wall, and exits the spongocoel through a larger opening referred to as the osculum. These sponges are considered to be asconoid. Asconoid sponges are usually relatively small sponges, being somewhat constrained by the efficiency of their canal system. Syconoid sponges possess ciliated radial canals contained within the body wall. Water flows into the dermal pores, and then enters the radial canals via small openings called prosopyles. At its interior end, the radial canal opens into the spongocoel. This opening is the apopyle. The largest sponges are leuconoid. Leuconoid sponges are the most common and generally the largest sponges. In leuconoids, there is no spongocoel per se. Dermal pores allow water to flow into incurrent canals which lead to flagellated chambers, where suspension feeding occurs. Excurrent canals carry water from the flagellated chambers to the osculum. apopyle radial canal prosopyle dermal pore syconoid body wall asconoid sponge leuconoid sponge 2 General Zoology Laboratory. Matthew K Nelson (2011) Obtain a Scypha sp. (Grantia sp.) whole mount slide. Observe the specimen with the light microscope under low power, then switch to high power. Draw and label the specimen. Classification The body forms of sponges reflected by the levels of complexity in canal systems do not seem to be indicative of relatedness. There are three extant classes of sponges: Class Calcarea - This group possesses calcareous spicules (CaCO3). The spicules are monaxon, triaxon, or tetraxon (straight, three pointed, or four pointed). These generally smaller marine sponges are most common in the tropics. The Calcarea include asconoid, syconoid and leuconoid forms. Class Hexactinellida - This class is referred to as the “glass sponges” because of the characteristic siliceous spicules (SiO2) of its members. The name “hexactinellida” refers to the six-pointed (hexaxon) spicules of glass sponges. These are all marine and either syconoid or leuconoid. Class Demospongiae - Most of the sponges are in this group. These are all leuconoid, but can be marine or freshwater. May have siliceous spicules or spongin, or both. Commercial bath sponges are made from members of this group Obtain a bath sponge specimen. This is the spongin matrix from an “ex-sponge.” Examine the sample under a dissecting scope. Freshwater sponges possess a special type of asexual reproduction that is used to survive freezing during overwintering. These sponges produce internal buds called gemmules (right), which consist of archaeocytes surrounded by tightly packed spicules. Archaeocytes eventually emerge from the gemmule and develop into new sponges. Obtain a prepared slide with a gemmule specimen. Observe under low and high power. Note the spicules if visible. 3 General Zoology Laboratory. Matthew K Nelson (2011) phylum Cnidaria The term Cnidaria ([knide = nettle] + [aria = like]) reflects the specialized stinging cells which cnidarians use for defense and prey capture. This group was once part of a larger group called the coelenterates, encompassing the Cnidaria and the Ctenophora (comb jellies). Since the coelenterates were not monophyletic, they were broken into two separate phyla. The body of a cnidarian is radially symmetrical. As a result, terms such as dorsal and ventral have no meaning for these organisms. Instead, relative anatomical positions are referred to with respect to the position of the mouth. The end of the body that is the location of the mouth is called the oral end, and the opposite end is the aboral. Organization Cnidarians possess incipient tissue-level organization. They are diploblastic, meaning that they possess two embryonic tissue layers. These two layers eventually develop into the two layers of the body: the epidermis and the gastrodermis. Between the epidermis and the gastrodermis is a gelatinous non-cellular layer referred to as the mesoglea. The epidermis comprises the outer layer of the body. Epidermal cells are contractile and can be used to change the shape of the body. Cnidocytes are also part of the epidermis. Cnidocytes are stinging cells unique to cnidarians, containing special stinging organelles called nematocysts. The gastrodermis lines the gastrovascular cavity, a chamber connected to the mouth. Cnidarians possess two body forms: polyp and medusa. These two body forms are found within the same species. Normally, the life cycle of cnidarians alternates between the polyp and medusa stage. Generally, the main function of the polyp is feeding, while the medusa is mainly concerned with sexual reproduction. The polyp is the sessile form. The aboral surface is usually attached to a substrate, while the oral surface is directed upward to allow for efficient feeding. The medusa is motile, swimming with the oral surface downward. In some groups, one or the other of these forms may be reduced or absent. Anemones, for example, do not have a medusa stage. Nematocysts The nematocysts of cnidarians possess a special tactile trigger called a cnidocil. When the cnidocil is engaged, the nematocyst is discharged, extending a long thread that punctures the prey item, injecting it with venom. The nematocysts of some cnidarians can deliver a serious sting to even a large organism such as a nosy human. Examine the demonstration slide of a discharged nematocyst. 4 General Zoology Laboratory. Matthew K Nelson (2011) Classification There are four classes of cnidarians. Class Hydrozoa - Freshwater or marine. Often colonial. Members usuallly possess both polyp and medusa phases. Class Scyphozoa - Most of the jellies. Generally, scyphozoans are medusae. The polyp is reduced in this group and usually exists as a scyphistoma, that produces medusae through strobilation. Class Cubozoa - Box jellies. polyp develops directly into medusa without strobilation. Class Anthozoa - Corals (colonial) and Anemones (solitary). mutualistic associations with single celled algae give these bright coloration. Hydrozoa Obelia (Order Hydroidea) The hydrozoan Obelia sp.