Platyhelminthes

Members of the phyla Platyhelminthes, Nematoda, and Rotifera have a vermiform (wormlike shape) and are the first in this manual to have a third layer of cells (the mesoderm) instead of just the epidermis and gastrodermis as in cnidarians. The mesoderm produces muscular layers, an excretory system, and an elaborate reproductive system.

The platyhelminths and aschelminths have bilateral symmetry and meet the environment in a more aggressive, head-on fashion than the radiate animals. Cephalization, a specialization of the anterior end of the animal, can be seen in the clustering of sensory structures and clumps of nerves (ganglia or "brains") in the head region. Many of the elaborations you will see are related to increases in mobility, higher activity, and general increase in responsiveness of the animals.

The body cavities of these and succeeding phyla have become more complex through the addition of specialized internal organs. The fluid in the body cavity may act as a hydrostatic skeleton, a site for the collection of wastes or gametes, and/or as a primitive circulatory system. Flatworms (Phylum Platyhelminthes) have their body cavities loosely filled with cells and are said to exhibit an acoelomate body plan.

Cnidarians are composed of only two cellular layers, an outer epidermis and an inner gastrodermis. In cnidarians these cell layers are thin and the animals are therefore very fragile. Animals above the cnidarians on the evolutionary tree have three layers of cells. Each layer gives rise to specific structures within the organism. The ectodermis is responsible for the production of much of the sensory system and surface skin (among other things), while the endoderm contributes to the growth of structures such as the lining of the gut. The third cell layer lies between the ectoderm and endoderm and is called the mesoderm). This third layer is used to build muscular, excretory, and reproductive systems. The mesoderm thus allows animals to become more organized in their structure. Cnidarians, for example, are organized mainly at the level of tissues, while those with three layers possess organs and elaborate organ systems.

There are three ways bodies can be organized around the cellular layers. The simplest organization (seen in the flatworms) is the acoelomate body plan. In these animals the body cavity is loosely filled with mesodermal cells called parenchyme. The pseudocoelomate body plan has a fluid-filled cavity called the pseudocoelom. Members of the aschelminth phyla are pseudocoelomate in structure. The final organizational level (seen in annelids and all other phyla above the aschelminths) is the coelomate body plan. Coelomates have the body cavity (the coelom) lined with mesodermal tissue (the peritoneum). The peritoneum surrounds all organs in the body cavity. In some coelomates the coelom may be present only in larval stages or may be greatly reduced. Click HERE for more information on body plans. Body cavities: Acoel, eucoel and pseudocoel.

Platyhelminths are commonly known as "flatworms," owing to their almost paper-thin structure. Their digestive system (if present) is incomplete, and they possess only a mouth. Flame cells are often present as part of an osmoregulatory system, and all members have a spongy body structure (an acoelomate body plan). Three classes will be covered in this laboratory. Members of one class (the turbellaria) are free-living, while the other two classes (trematoda and cestoda) are obligate parasites.

FIGURE 1. STRUCTURE OF A TURBELLARIAN (Dugesia sp.).

Planarian Nervous System

Figure 1b. Planarian Cross Section

Class Trematoda: Parasitic Life Cycle (Fig 2). Eggs pass out with the primary host's feces and eventually contaminate water, where they are ingested by snails (the first intermediate host). The eggs hatch within the snail's gut and develop into miracidia that burrow into the walls of the snail's digestive tract. Within these tissues the miracidium is transformed into a sporocyst. The sporcyst develops internal chambers; and, within each chamber, a redia develops asexually. Each of the redia is also chambered and asexually produces a number of internalized cercaria. The cercaria have finished their development within the first intermediate host, so they then burrow out of the snail and become free-swimming. Thus, if a single miracidium is ingested by a snail, several hundred cercaria can be liberated. The cercaria then seek out a second intermediate host (a fish) and burrow through the fish's skin. Once in the fish's muscles they lose their tails and encyst (the encysted form is called a metacercaria). When raw or poorly cooked fish is eaten the young flukes are released from their cysts by digestive juices in the stomach. Once out of the stomach they make their way up the common bile duct to the liver where they attach and suck the host's blood. In some areas of China the incidence of parasitism in man is close to 100%. What adaptations can you list for the liver fluke's parasitic lifestyle? Do you like sushi?

FIGURE 2. PARASITIC LIFE CYCLE OF THE CHINESE LIVER FLUKE. The adults live in the bile ducts of the primary host (human). Two intermediate hosts are infected by larvae: snails (the first intermediate host) and fish (second intermediate host).

FIGURE 3. MAJOR ORGANS AND SYSTEMS IN AN ADULT CHINESE LIVER FLUKE (Opisthorchis sinensis).

FIGURE 4. MAJOR ORGANS AND EXTERNAL STRUCTURES ALONG THE LENGTH OF A TAPEWORM (CLASS CESTODA).

Turbellaria	Detail of the ventral surface of Dugesia showing the epithelium and cilia on its surface.	Detail of the lateral margin of the body of Dugesia showing the adhesive gland on the ventral surface.	Major anatomical features of a planarian
Cross section of a planarian (anterior) showing the intestine	Cross section of a planarian in the region of the pharynx	Cross section of a planarian (posterior) showing the intestine	Pigmented eye cups of the flatworm Dugesia.
Nervous system of Dugesia	Nervous system of a turbellarian, colour has been added	Bdelloura candida Detail of the anterior end showing the brain, eyespots and the longitudinal nerve cords. Anterior branches of the intestine are also visible.	Flame cell - protonephridia.
Major anatomical features of a planarian - colours have been added to highlight the different systems	Diagram showing the major features of the flatworm Bdelloura candida - systems are shown in colour.
Trematoda	Fasciola hepatica, Sheep liver fluke	Trematoda - Fasciola hepaticus - Diagram of the adult sheep liver fluke with major systems shown in colour.	Egg of a trematode life cycle (Chlonorchis sinesis?)
Chlonorchis sinesis - Sporocyst containing redia	Chlonorchis sinesis - Redia stage in a trematode life cycle	Schistosoma - Cercaria of the the shistosoma parasite	Chlonorchis sinesis - Metacercaria
Trematoda - Clonorchis sinensis - Chinese liver fluke showing major anatomical features.	Trematoda - Clonorchis sinensis - Diagram showing the major anatomical features of the chinese liver fluke. Major systems have been highlighted with colour	Trematoda- Labeled slide of Chinese liver fluke	Trematoda - Clonorchis sinensis - Anterior end of the chinese liver fluke showing the mouth, intestine and ventral sucker
Trematoda - Clonorchis sinensis - Uterus of the chinese liver fluke containing mature eggs.	Trematoda - Clonorchis sinensis - Middle section of the chinese liver fluke showing the ovary shell gland and seminal receptacle.	Trematoda - Clonorchis sinensis - Posterior end of the chinese liver fluke - testes	Trematoda - Schistosoma mansoni - Male and female schistosome
Cestoda - Tapeworm.	Cestoda - Scolex of a tapeworm.	Cestoda - Portion of a mature proglottid.