Ch. 6: Cestodes and Tapeworms

General Characteristics of Adult Tapeworms

• Exclusively inhabit the intestinal tract of their hosts.
• Possess no digestive system, relying on absorption through their skin.
• Exhibit heteroxenous life cycles, often involving multiple hosts.

Body Structure of Adult Tapeworms

• Comprised of three main parts: scolex (head), neck, and strobila (body).
• Scolex includes structures for attachment to the host, while the strobila is formed of proglottids.

Scolex

• The anterior end of the tapeworm, equipped with suckers and hooks for secure attachment to the host's intestinal wall.

Neck

• The region responsible for the formation of new proglottids, with older proglottids located towards the posterior end.

Strobila

• A series of segments (proglottids) that makes up the main body of the tapeworm.
• Each proglottid is hermaphroditic, containing both male and female reproductive organs.

Proglottid

• Represents an individual segment within the strobila that houses a complete and sexually mature reproductive system.

Polyzoic vs. Monozoic

• All medically significant tapeworms are polyzoic, possessing multiple proglottids.
• Monozoic tapeworms consist of a singular proglottid.

Strobilation

• The process where new proglottids emerge from the neck region, promoting growth and development.

Copulation Methods

• Can self-fertilize, cross-fertilize with other proglottids from the same worm, or mate with other worms by intertwining their bodies.

Fate of Proglottids

• Proglottids can detach through apolysis, pseudoapolysis, or hyperapolysis, affecting egg release.

Apolysis

• Involves proglottids detaching completely and either passing through the host intact or disintegrating to release eggs.

Pseudoapolysis

• Eggs are released from proglottids through the uterine pore while the worm remains in the host's intestines.

Hyperapolysis

• Early detachment of a proglottid before the formation of eggs occurs.

Pseudoscolex

• Refers to folds of the strobila contributing to the worm's structural integrity.

Sucker-like Organs on Scolex

• Includes acetabula (muscular and cup-shaped), bothridia (leaf-like, mobile margins), and bothria (shallow sucking grooves).

Acetabula

• Typically arranged in groups of four; common in many tapeworm species.

Bothridia

• Usually organized in fours; feature adaptable, mobile margins for effective adherence.

Bothria

• Comprised of 2 to 6 shallow grooves; most commonly consist of two.

Nervous System Complexity

• Bothridia are associated with the most intricate nervous system, featuring ganglia and sensory organs.

Cestode Tegument Characteristics

• Microtriches enhance surface area for nutrient absorption.
• Syncytial structure presents a contiguous layer of cytoplasm, devoid of separating cell membranes.
• Glycocalyx, a carbohydrate layer, covers microtriches and aids in nutrient absorption.

Functions of Glycocalyx

• Enhances enzymatic activities of amylase in the host.
• Aids in nutrient absorption, particularly bile salts.
• Inhibits digestive enzymes like trypsin and lipase, protecting the worm's tissues.

Calcareous Corpuscles

• Scattered throughout larval stages, these structures act as buffers against organic acids from metabolic waste.

Nervous System Features

• Simplistic worms possess bothria with one pair of cerebral ganglia.
• More complex species have acetabula with multiple pairs of longitudinal nerves.

Flame Cells

• Specialized cells for filtering excess water, operating through excretory canals running along the strobila.

Reproductive Characteristics

• Most tapeworms are monoecious with some dioecious; each proglottid contains reproductive organs.

Protandry

• A type of sequential hermaphroditism where an organism first matures as a male and later transitions to female.

Protogyny

• Involves organisms born as females, possessing female reproductive organs from the outset.