Drawings and a few questions need to completed. all information is provided in the document
LABORATORY PRACTICAL 1 1 BIOL2151 2020 - ONLINE LABORATORY PRACTICAL 1 PHYLUM CNIDARIA Introduction The cnidarian body consists of a central blind sac, the gastrovascular cavity (formerly called the coelenteron), enclosed by a body wall comprising two epithelia, the outer epidermis and the inner gastrodermis (Hickman Fig. 7.3). A gelatinous connective tissue layer, the mesoglea, lies between the two epithelia. The mouth opens at one end of the gastrovascular cavity and marks the oral end. The mouth is at the tip of a process, the manubrium, that elevates it above the oral surface. The opposite pole is the aboral end. The imaginary line connecting the oral and aboral poles is the axis of symmetry around which the radial symmetry of the body is organized. The mouth is usually surrounded by one or more circles of tentacles. The defining cnidarian feature is, of course, possession of stinging cells, or cnidocytes (Figs. 7.4, 7.6). Characteristic of the epidermis, they are also sometimes found in the gastrodermis. Cnidocytes contain an explosive organelle, the cnida, which, upon proper stimulation, inverts and ejects a slender, often barbed and toxic thread in the direction of prey or predator. Three types of cnidae are found in cnidarians. Most cnidae are nematocysts, which are toxic, and these are present in all three higher cnidarian taxa. The basic body plan described above can be manifest as a swimming medusa or attached polyp. In some taxa only one generation is present whereas in others both are found. A life cycle featuring alternation of sexual, swimming medusae with benthic asexual polyps is typical of many cnidarians. All cnidarians are carnivores feeding on live prey which they usually capture using tentacles armed with cnidocytes. Digestion occurs in the gastrovascular cavity which is typically equipped with ciliated canals for distribution of partly digested food. Cnidarians are ammonotelic and diffusion across the body and tentacle surface eliminated the ammonia from the body. Gas exchange is across the general body surface. The nervous system is a plexus of basiepithelial neurons serving sensory and motor systems (Fig. 7.5). Most cnidarians are dioecious (separate sexes). The life cycle typically includes a planula larva. Cnidarians are chiefly marine but the well-known Hydra is an exception. Review the taxonomic Classes and their traits shown in Fig. 7.1. Note that we will not cover Classes Staurozoa or Myxozoa in prac. Note also that myxozoans are highly modified cnidarians,that have exceptions to some of the traits described above. Class Hydrozoa Hydrozoa is a diverse taxon of about 3000 species of mostly marine cnidarians. Cnidocytes are present only in the epidermis, not in the gastrodermis. The life cycle usually includes both polyp and medusa generations (Fig. 7.10) but may be entirely polyp or entirely medusa. Polyps typically are colonial and medusae usually solitary. Some form colonies of combinations of polyps and medusae. The few freshwater cnidarians, such as Hydra, are hydrozoans. Hydrozoan polyps are usually small, about 1 mm in length, and colonial. Hydromedusae are also small, at least in comparison with scyphomedusae, and are usually less than 1 cm in diameter (Fig. 7.12). Hydromedusae are further distinguished from scyphomedusae by possession of a velum, a circumferential shelf of tissue that encircles the subumbrellar concavity and functions as an adjustable diaphragm to create a pulse of water for swimming. Solitary Species Hydra is a freshwater species in which the medusoid generation is absent and the polyps solitary (Figs. 7.4, 7.8, 7.9). As in most cnidarians, food is captured by the tentacles, stung with the cnidocytes, and then transferred to the mouth and gastrovascular cavity for initial extracellular digestion. This is followed by intracellular digestion as molecules and food particles are distributed by gastrodermal ciliary currents in the gastrovascular cavity and then endocytosed by gastrodermal cells. 1.Examine the textbook photos and drawings of Hydra (Hickman Figs. 7.4, 7.8, 7.9). Then examine the image below of a whole mount of a preserved Hydra. Compare it to the textbook figures; find the body structures shown in Fig. 7.4; notice the bud developing on the side of the body column, similar to Fig. 7.8; and notice the bumps on the tentacles which are nematocysts. Now watch this short video that shows live Hydra, including movement and feeding. https://www.youtube.com/watch?v=dFvJTmjUOyU Examine the image below of a Hydra c.s. (cross section). Make your own drawing of it beneath the image, and label the structures you can identify from Fig. 7.4. This is a cross section of the body column below the mouth, so you can expect to see the gastrovascular cavity, epidermis, gastrodermis, and mesoglea. Insert your labelled drawing here: Colonial Species Obelia consists of colonies of numerous polyps on branched stalks (Fig. 7.10). It has a life cycle in which polyp and medusa are of approximately equal importance. 2.Examine the images below of a prepared slide of an Obelia colony WM (whole mount). A creeping, rootlike stolon lies on the surface of a firm substratum and serves as anchor and base for the entire colony. Commercially prepared slides include only a small piece of the colony and the stolon is rarely present. Numerous erect stems arise from the stolon. The stems branch in patterns characteristic of the species and bear polyps. The stems, stolons, and polyps are enclosed in a transparent, chitinous exoskeleton called the perisarc. This is a non-living secretion of the epidermis. The hollow interior of the animal is the gastrovascular cavity. The cavity extends uninterrupted throughout the colony and is both a chamber for extracellular digestion and a fluid transport system to deliver partially digested food through the colony. Obelia polyps (= zooids) are dimorphic, with two types in the colony. Gastrozooids are responsible for feeding, which they do by capturing and ingesting zooplankton. The polyp is enclosed in a bell-shaped expansion of the perisarc called the hydrotheca. The oral disc and tentacles together are called a hydranth (= water flower). Gonozooids are reproductive and produce medusae by asexual budding. The medusae break free to swim out into the sea. Make a drawing of part of the Obelia colony showing several polyps, labelling its structures as in Fig. 7.10. You can insert the drawing on the next page. Insert your labelled drawing here: 3.Diversity of Hydrozoa. Examine the 3 additional colonial hydrozoans below that have different body plans, including the photos shown in lecture (left, from Hickman) and the specimens from our lab (right). Choose one of these specimens to write about on the next page, explaining one key feature you can see that identifies it as a cnidarian. Portugese Man-of-War, Physalia. This is a pelagic colony of modified polyps. The gas filled float supports trailing gastrozooids, dactylozooids and gonozooids. By-the-Wind Sailor, Velella. This is also a drifting colony. Note the float with its sail is a gastrozooid, and there are gonozooids and dactylozooids underneath it. Calcareous skeleton of Millepora. This is a member of the small group of hydrocorals, where the colonial polyps secrete a calcareous skeleton. Write your answer here (it can just be a couple sentences): Class Anthozoa This heterogeneous taxon includes stony corals, sea fans, anemones, sea pansies, and others. Anthozoa is the largest cnidarian taxon, with over 6000 species of exclusively marine polyps. Anthozoans are always polyps, with no medusa in the life cycle. The polyps can be solitary or colonial. The polyps tend to be large, with extremes up to 1 m in diameter, and colonies such as the corals may be even larger. The mouth opens into a flattened, ectodermal invagination known as the pharynx, which in turn opens into the gastrovascular cavity (Fig. 7.22). Many have endosymbiotic zooxanthellae that figure importantly in their biology. The gastrovascular cavity is compartmentalized by longitudinal septa. Anthozoans are divided in the subclasses Octocorallia (septa in multiples of 8, and 8 pinnate tentacles) and Zoantharia or Hexacorallia (septa in multiples of 6, with numerous tentacles that are not pinnate). The Octocorallia includes the organ pipe corals, the soft corals, the gorgonian corals, and the sea pens. The Zoantharia is the larger subclass and includes the sea anemones and the hard (or stony) corals. Subclass Octocorallia 4. Examine the skeleton below of a Gorgonian coral from our lab. The polyps are retracted, but may be seen as small bumps on the skeletal mass. Look at Fig. 7.27 for reference. Examine the sea pen, Pennatula, below from our lab. The central stalk is a modified polyp, and the feeding polyps sit at the tips of the lateral fringes. Compare it with a living animal, shown in Fig. 7.21. Now look more closely at the Gorgonian coral on the previous page, and find evidence that it is an octocoral as opposed to a hexacoral. Find a polyp on which you can see 8 tentacles, and circle it and label it on the photo. Subclass Zoantharia (or Hexacorallia) Sea Anemones The sea anemones are solitary and have the morphology typical of anthozoan polyps (Fig. 7.22). Anemones can dramatically alter their size and shape by inflating (with seawater) or deflating. About 1350 species are known. Most anemones attach to a firm substratum by the pedal disc, but many inhabit soft substrata and do not attach. The periphery of the gastrovascular cavity is partitioned by abundant longitudinal, sheetlike septa which greatly increase the surface area for absorption of food and its subsequent intracellular digestion. The central region of the cavity is unpartitioned and is the site of extracellular digestion. The free edges of the septa border the central region and secrete hydrolytic enzymes into it. Polymorphism and alternation of generations are absent. Anemones are dioecious or consecutive hermaphrodites. 5. Watch the video on the Canvas module page of live sea anemones from our lab, and look for the external structures shown in Fig. 7.22. Canvas video 1 - sea anemone feeding Watch as Angela releases brine shrimp ("sea monkeys") from a pipet for the anemones to capture. At 0:58, you'll see a shrimp swim toward the tentacles, which capture it at 1:04. Another one is captured at 2:15. Observe the movements of the tentacles and body, and think about which muscle fibers contract to create those movements (circular and/or longitudinal fibers, and in what part(s) of the body). To investigate the feeding response, watch this video on the Canvas module page: Canvas video 2 - sea anemone nematocyst response Angela first presses a clean wooden stick against the tentacles. Notice that the tentacles recoil from the disturbance, but they don't