Question

Which class of Platyhelminthes includes the tapeworms?

• A. Turbellaria
• B. Trematoda
• C. Cestoda
• D. Monogenea

Answer 1

Answer:

C. Cestoda

Understanding Platyhelminthes and its Classes

• The phylum Platyhelminthes, commonly known as flatworms, encompasses a diverse group of invertebrates.
• They are characterized by being acoelomate (lacking a body cavity), triploblastic (having three germ layers: ectoderm, mesoderm, and endoderm), and exhibiting bilateral symmetry.
• Most platyhelminths are parasitic, but some are free-living, inhabiting aquatic and moist terrestrial environments.
• This phylum is broadly divided into several classes, primarily based on their lifestyle (free-living or parasitic) and morphological features.

Class Cestoda: The Tapeworms

• The class Cestoda exclusively comprises the highly specialized endoparasites known as tapeworms.
• Tapeworms are unique among flatworms for their distinctive body structure, which includes:
  • A scolex at the anterior end, equipped with suckers and/or hooks for attachment to the host's intestinal wall.
  • A short neck region.
  • A long, segmented body called a strobila, composed of numerous repeating units known as proglottids.
• Each mature proglottid is essentially a reproductive factory, containing both male and female reproductive organs (hermaphroditic).
• A remarkable adaptation of Cestodes is the complete absence of a digestive system. They absorb digested nutrients directly from the host's intestine through their highly permeable body surface (tegument).
• Their life cycles are typically complex, involving one or more intermediate hosts and a definitive host.
• Common examples of tapeworms important for competitive exams include:
  • Taenia solium (pork tapeworm), which can cause taeniasis and, more severely, cysticercosis in humans.
  • Taenia saginata (beef tapeworm), causing taeniasis.
  • Echinococcus granulosus (dog tapeworm), responsible for hydatid disease in humans, forming large cysts primarily in the liver and lungs.

Other Important Classes of Platyhelminthes

• Turbellaria: This class largely consists of free-living flatworms, commonly found in freshwater, marine, and moist terrestrial environments. Planarians are a well-known example, notable for their remarkable regenerative abilities. Unlike parasitic classes, they possess a ciliated epidermis and a simple digestive system.
• Trematoda: Commonly known as flukes, these are all parasitic, primarily endoparasites. They are characterized by having an oral sucker and often a ventral sucker (acetabulum) for attachment. They have an incomplete digestive system. Important examples include:
  • Schistosoma spp. (blood flukes), causing the disease schistosomiasis (bilharzia), which has a complex life cycle involving snails as intermediate hosts.
  • Fasciola hepatica (sheep liver fluke), which infects the liver and bile ducts of various mammals, including humans.
• Monogenea: These are primarily ectoparasites, mostly found on the skin or gills of fish, and have a direct life cycle (requiring only one host). They possess a complex posterior attachment organ called an opisthaptor.