Why can tapeworms grow very long while remaining thin?

Tapeworms can grow to significant lengths while remaining thin because they lack a traditional digestive tract, circulatory system, or respiratory organs. Instead of relying on complex internal systems to process food or transport nutrients, they absorb all necessary nourishment directly through their body surface, or tegument.123

This specialized tegument is metabolically active and features microscopic, spinelike structures called microtriches (or microvilli), which increase the surface area available for the absorption of amino acids and sugars from the host's environment. Because the tapeworm's body is thin and flattened, every cell remains close to the surface, allowing for efficient nutrient uptake and gas exchange via diffusion without the need for specialized transport systems.4312

Furthermore, the tapeworm's body, or strobila, is composed of a series of repeating units called proglottids. These segments bud off from the neck region, allowing the worm to extend its length while each segment functions as a self-sufficient reproductive unit.415