BIOL 202 LAB 8

Invertebrates: Porifera, Radiata & Acoelomates

PORIFERA The phylum Porifera includes around 10,000 species of sponges. Despite this seemingly large number, all sponges share many anatomical similarities which tie in to their common lifestyle. Sponges are sedentary animals that lack tissues or organs—characteristics that cause many zoologists to classify them as an outgroup to the evolutionary line that leads to all other modern animals. Their “bodies” consist of 4 primary cell types arranged in loose aggregates around a system of pores and canals through which sponges pass water. While most species are marine, there are around 150 freshwater species that have been described. 1. Obtain a prepared slide of a longitudinal-section through Grantia, Scypha or

another sycon sponge. As you view the slide, try to visualize the path of water flowing from the outside of the sponge toward the inside. Water flow is crucial to understanding the anatomy of sponges and the reasons for their cellular organization. Since sponges do not move, every aspect of their existence, from feeding to reproduction, hinges on water moving through them.

2. The most obvious landmark on your slide is the large central cavity called the

spongocoel that passes through the center of the sponge. Before reaching the spongocoel, water passes through a complex series of chambers and canals in the sponge and, in doing so, is stripped of most food particles and oxygen it contains.

3. Water enters the sponge through pores on the body surface called ostia. Ostia

channel water down the incurrent canals to a larger number of tiny pores scattered along the folds of the incurrent canals (see Figures below).

4. Specialized cells called choanocytes lining the interior surface of the radial canals

trap small food particles with their flagella and engulf them through phagocytosis. The beating of the choanocytes’ flagella actually creates the water currents that flow through the sponge body.

5. Finally the water is pushed out of the large osculum which is usually located at the

top of the sponge. One of the ways sponges reproduce is by shedding gametes (much like sea urchins you studied in the previous chapter). When water flows out of the osculum, gametes may be carried out and collected by nearby sponges as water flows into them.

6. Now examine preserved specimens and prepared slides of sponges. Try to identify

some of the structures on the next page. Note: not all structures will be visible.

PHYLUM CNIDARIA The phylum Cnidaria is composed of three major classes of mostly marine carnivores: Hydrozoa (Hydra, Obelia, Portuguese man-o-war), Scyphozoa (true jellyfish) and Anthozoa (sea anemones and colonial corals). Of the ~10,000 aquatic species estimated to exist in this phylum, the majority are marine. All cnidarians are radially symmetrical and most are metamorphic, meaning they have different body forms during their lives. The polyp form is generally represented by a cylindrical organism which remains attached to the substrate by a short stalk. The medusa is a more circular, free-swimming form resembling the familiar jellyfish in morphology (Fig. 9.1). The classes of cnidarians are distinguished primarily by the dominance of the polyp or medusa stage in the lifecycle. In hydrozoans the polyp form predominates the lifecycle, while in scyphozoans it is the medusa form which predominates. Anthozoans exist only as polyps; the medusa stage has been completely lost. Another universal characteristic of this phylum is the presence of tentacles armed with stinging cells used for defense and for capturing food. These cells are called cnidocytes, from which term the phyllum derive its name. Unlike sponges, the cnidarian body is arranged into two discrete tissue layers, giving them a diploblastic arrangement. There is an outer epidermis and an inner gastrodermis. Sandwiched between these two cellular layers is an inert, gelatinous substance called mesoglea. Thus cnidarians possess a more complex organization of their cells than sponges and are consequently capable of many more intricate behaviors as a result of the specializations of their two cell layers.

Hydrozoan Anatomy Most members of this class undergo a regular alternation of generations between body forms that utilize asexual reproduction (polyp form) and sexual reproduction (medusa form). In general the polyp is the predominant body form, and in many groups, polyps are assembled into colonies, as in the case of Obelia. Members of the genus Hydra are unusual in that they do not produce medusae. Rather Hydra exist as single, mobile polyps that reproduce either sexually (through production of sperm and eggs) or asexually (through budding). In either scenario a new polyp is the result. Solitary Hydrozoans 1. Examine prepared slides of Hydra. These small hydrozoans exist as polyps in

shallow, freshwater ponds and streams. Their diminutive size is deceptive, for they are fierce predators of small aquatic invertebrates as they sit motionless among the submerged rocks, twigs and vegetation with their tentacles outstretched waiting for prey to pass too closely. Using a microscope, examine the w.m. slide and locate the following structures: tentacles, basal disc, bud (may not be present), gonads (ovaries or testes). Use the figures shown in the lab manual and Table 9.1 to study Hydra anatomy.

2. If available, examine a longitudinal section (l.s.) through Hydra. In addition to the

structures visible on the whole mount, locate the following: mouth, gastrovascular cavity (coelenteron), epidermis, gastrodermis, mesoglea. cnidocyte.

Table 9.1 • Anatomy of Hydra Structure Function

Tentacles Defense and prey capture

Mouth Ingestion of food and elimination of indigestible particles (egestion)

Bud Product of asexual reproduction; will fall off when mature and become a self-sufficient organism

Gonads (testes and ovaries) Organs for sexual reproduction; Hydra are dioecious, meaning that an organism has either testes or ovaries, but not both (i.e., male or female).

Basal disc (base) Specialized region for attachment to the substrate

Colonial Hydrozoans Now turn your attention to a more typical member of the class Hydrozoa. Members of the genus Obelia are colonial hydrozoans that are connected by branches of a common gastrovascular cavity (coenosarc), making them all part of a larger functioning body. Due to this cooperative venture, certain polyps develop into highly specialized feeding polyps, while others lose the ability to feed altogether in exchange for an enhanced ability to reproduce. Despite its appearance, Obelia is not a true colony since the numerous polyps all form from the repeated budding of a single individual. 1. Obtain a slide of a colony of Obelia polyps. Examine it using a low power and

identify the structures in the following figure below and defined in Table 9.2: hydranth (feeding polyp), tentacles, hypostome, gonangium (reproductive polyp), medusa buds, perisarc and coenosarc.

2. Now examine a w.m. slide of an Obelia medusa using medium power. Locate the

following structures which are defined in Table 9.2: tentacles, manubrium, mouth and gonads. The medusa stage of most hydrozoans is typically a very short-lived stage devoted primarily to reproduction. Male and female medusae release haploid sperm and eggs, which fuse to form a diploid zygote. This zygote develops into a swimming planula larva which settles to the bottom of the ocean floor where it develops into a new polyp. Through this mechanism, Obelia has an alternation of generations from polyp to medusa and back to polyp again in a continuous cycle.

Table 9.2 • Anatomy of Obelia Body Form Structure Function

Polyp Hydranth (feeding polyp) Polyp specialized for food acquisition

Tentacles Defense and prey capture

Gonangium (reproductive polyp) Polyp specialized for reproduction

Hypostome Elevated mound of tissue which expands or contracts to regulate size of mouth opening

Medusa buds Product of asexual reproduction; medusae will be released from the gonangium when mature and will produce either sperm or eggs which fuse with the respective gamete forming a zygote which will develop into a new polyp

Coenosarc Common chamber within which extracellular digestion occurs; nutrients are distributed throughout organism

Perisarc Translucent outer covering of organism; serves protective function

Medusa Tentacles Defense and prey capture

Manubrium Stalk of fleshy tissue which supports the mouth

Mouth Ingestion of food and egestion of indigestible particles

Gonads Organs for sexual reproduction; either testes or ovaries

Scyphozoan Anatomy Members of the class Scyphozoa are called jellyfish because of their thick layer of gelatinous mesoglea. In this class, the medusa stage dominates the life-cycle, with the polyp stage being relegated to an inconspicuous, short-lived larval form which quickly matures into a polyp that buds off young medusae. Aurelia, the moon jellyfish, is a common, widely-distributed genus within this class. View prepared slides if available

ACOELOMATES Animal Body Plans Animals, such as flatworms, containing three layers of tissue are known as triploblastic; remember cnidarians are diploblastic and sponges have no true tissues. In fact most animals with which we are familiar are triploblastic. Within the triploblastic animal species different developmental strategies have evolved for packaging the tissues and compartmentalizing the body space. As a result, three different body plans have arisen; acoelomate, pseudocoelomate and eucoelomate. Body Plans of Triploblastic Animals Acoelomate—animals whose central space is filled with tissue (mesoderm). No true

body cavity exists. Example: flatworms. Pseudocoelomate—animals with a central body cavity that lies between gastrodermis

and mesoderm. Example: roundworms (nematodes). Eucoelomate—animals with a central body cavity that lies within mesoderm. Examples:

earthworms, molluscs, insects, chordates. Remember that the gut (or digestive cavity) does not count as a body cavity when determining the type of coelom that an animal possesses. All triploblastic animals possess a digestive cavity. Some, such as pseudocoelomate and eucoelomate animals, possess an additional space within the body—the coelomic space. Others, the acoelomates, lack this additional body cavity. PHYLUM PLATYHELMINTHES There are an estimated 15,000 species of flatworms. They range in size from a few millimeters in length to over 20 meters long! Despite this wide range in length, flatworms are never more than a few millimeters thick. Their anatomy differs substantially from the cnidarians you observed in the previous chapter. In addition to the epidermis and gastrodermis, flatworms possess a third tissue layer sandwiched between the two called mesoderm. During embryonic development this third tissue layer differentiates into muscles (among other things), making flatworms one of the first highly motile groups of animals. Flatworms have bilateral symmetry, and free-living species typically possess a concentration of nervous tissue and sensory structures at the cranial end of the body—a condition known as cephalization. Bilateral symmetry is related to the animal’s tendency when swimming or crawling to keep the same end of the body forward and the same surface downward toward the substrate. When an animal consistently moves with the same part of the body forward, there is an evolutionary tendency for its descendants’ sensory organs and nervous system to become concentrated at the forward end where environmental conditions are first being met. Likewise the mouth generally exists in this region where it can ingest food that has been detected by the sensory organs. Although the latter feature is not the case with flatworms, they do have extensive organ development and display several organ systems. Despite these similarities, members of each class occupy radically different niches; thus natural

selection has molded them in different ways to produce specific adaptations to these unique lifestyles. Turbellarian Anatomy Members of the class Turbellaria are free-living flatworms that typically inhabit freshwater streams and ponds, oceans and moist terrestrial environments where they may be found under submerged rocks, foliage and moist debris. 1. Obtain a w.m. slide of the common planaria (Dugesia) and view it using low power. 2. Locate the cranial (anterior) end of the body where you will see a conglomeration

of sensory structures including eyespots and auricles. The eyespots of a planaria sense light (only shadows and direct light, not images) and the auricles are chemoreceptors that detect dissolved chemicals in the water. Together these sensory structures provide information about the outside world to the planaria’s brain and ladder-like nervous system (not visible on the slides).

3. You should also notice the darkly-stained, highly-branched gastrovascular cavity

that spreads throughout the entire body. Since planaria lack a circulatory system to deliver nutrients throughout the body, their gastrovascular cavity must reach throughout the body to minimize the distance for nutrients to diffuse from the gastrovascular cavity to the neighboring tissues.

4. A long, clear tube should be visible near the middle of the planaria. This is the

pharynx, a muscularized extension of the gastrovascular cavity that is extended from its sheath in the body for feeding.

5. Obtain a c.s. slide through a planaria and view it using medium power. If you have

several cross-sections on your slide, choose one through the middle region of the planaria that captures the tubular pharynx.

Planaria possess a simple excretory system (not visible on the slide) consisting of small structures called flame cells that are situated at the terminal ends of a branching network of tubules. These tubules collect metabolic wastes from nearby tissues through diffusion and the flame cells create a current that pushes these wastes along the tubule system and out of the animal through tiny pores distributed along the body surface. The development of an excretory system designed to collect, concentrate and eliminate metabolic wastes was a major evolutionary step which was one of the prerequisites for animals to evolve larger body sizes.

Trematode Anatomy Members of the class Trematoda are the parasitic flatworms commonly referred to as flukes. They generally have complex lifecycles often involving several host species. Flukes do not obtain the incredible sizes for which some species of tapeworms are notorious; the largest flukes only measure about 3 inches in length. As parasites, flukes display several specific adaptations for this lifestyle. Suckers are present for attachment to the host’s inner body wall or organs and flukes are surrounded by a thin, protective cuticle that prevents the digestive enzymes of the host from dissolving them. Another major adaptation of flukes (and other parasites like tapeworms) is their prolific production of eggs. Flukes are essentially reproductive factories, continuously churning out thousands of eggs a day for many years. Even with such huge numbers, only a tiny percentage of these eggs will make it out of the host, hatch into free-swimming larvae, find another suitable host, mature, leave that host’s body, find another host and reproduce to complete the lifecycle. 1. Obtain a w.m. slide of a trematode (either Fasciola or Clonorchis) and view it using

low power. Members of these two genera are liver flukes. 2. Identify the following structures depicted the following figures and defined in Table

10.1: oral sucker, pharynx, gastrovascular cavity, ventral sucker, genital pore, testes, ovary, uterus and excretory pore.

Notice that flukes possess both testes and an ovary. Most flukes are monoecious and are capable of self-fertilization, another adaptation to the oftensolitary lifestyle of an endoparasite, a parasite which lives inside its host. Slides to Examine: ZD 1-22 , ZD 1-1, ZD 3-1, ZE 4-13

Table 10.1 • Anatomy of a Liver Fluke Structure Function

Oral sucker Specialized for attachment to host; used in feeding

Pharynx Muscularized tube for pumping in blood and body fluids from host

Gastrovascular cavity Forked tube for digestion (minimal in flukes) and distribution of nutrients throughout body

Ventral sucker Secondary point of attachment to host

Genital pore Receives sperm produced in testes and moves it to the uterus where eggs may be self-fertilized

Testes Produce sperm

Ovary Produces eggs

Uterus Fertilized eggs develop here

Excretory pore Releases metabolic waste products out of the body

Cestode Anatomy Members of the class Cestoda, commonly referred to as tapeworms, represent the most specialized group of parasitic flatworms. They have many remarkable adaptations that allow them to be successful endoparasites (parasites that live within the body of their host). Like trematodes, they inhabit the gut of a host and possess a thin cuticle to prevent being digested. However, tapeworms completely lack a mouth and gastrovascular cavity—nutrients are absorbed directly through the cuticle of each proglottid composing the animal’s body. The scolex, or head, is modified for attachment to the host and usually possesses an array of hooks and suckers that embed the cranial end of the tapeworm deep inside the intestinal wall of the host. The remaining chain of proglottids trails behind, bathed in the intestinal juices of the host digestive tract. Tapeworms grow by producing new proglottids behind the scolex. As the proglottids mature, they enlarge and are shifted toward the rear of the tapeworm as new proglottids are continually being produced ahead of them. Proglottids at the terminal end of the body containing fertilized eggs drop off the tapeworm and the eggs are passed out of the host with feces. Although each proglottid represents a functionally separate reproductive unit, all of the proglottids are connected by a common series of excretory canals and longitudinal nerves. 1. Obtain a slide of proglottids from the tapeworm Taenia pisiformes, the dog

tapeworm. Examine the slide using low power. (Slide: ZD 4-12)

2. On the cranial section of tapeworm identify the following structures defined in Table 10.2: scolex, hooks, suckers, neck, immature proglottids. Refer to your textbook for some example figures.

Table 10.2 • Anatomy of a Tapeworm Structure Function

Scolex Cranial end of tapeworm; lacks sensory structures but possesses modifications for attachment to intestinal wall of host

Hooks and suckers Modified structures on scolex for attachment to host

Neck Constricted portion signifying caudal end of scolex; marks the site of origin of immature proglottids

Immature proglottid Newly produced segment of the tapeworm that has undeveloped reproductive organs

Excretory canals Longitudinal channels running along the outer margins of the body that deliver metabolic waste products out of the tapeworm

Mature proglottid Tapeworm segment that has functional reproductive organs

Uterus Site of fertilization and early embryonic development of eggs

Ovaries Produce eggs

Vagina Point of entry into female reproductive tract through which sperm travel to reach the eggs; joins with the genital pore

Testes Produce sperm

Ductus deferens (vas deferens) Canal through which sperm pass as the exit the proglottid through the genital pore

Genital pore External opening common to the male and female reproductive tracts of the tapeworm; sperm exit and enter proglottids through this opening