Final answer:
Sea cucumbers and sea urchins have long, coiled guts to aid in digestion and nutrient absorption due to their diets consisting of particles and detritus, which require extensive digestion.
Step-by-step explanation:
The sea cucumbers and sea urchins need long, coiled guts primarily to aid in digestion and absorption of nutrients. This is because both sea cucumbers and sea urchins have diets that consist largely of particles or detritus found in the sand or water, which require extended digestion due to their composition. Sea cucumbers, for instance, are deposit feeders, and they consume a variety of organic matter scattered across the ocean floor. Their elongated digestive systems allow more time for the breakdown of these complex food sources and the absorption of nutrients that are essential for their survival.
The sea urchins, being omnivorous grazers, consume algae, decomposing material, and other small organisms found on coral reefs and rock surfaces. The lengthy gut facilitates the digestion of a diverse diet, making adequate nutrient absorption possible. Since echinoderms have a simple open circulatory system with no heart, and a nerve net rather than a centralized brain, efficient digestion and nutrient absorption become even more critical for their survival. The intricate structure of their digestive system optimizes the available surface area for digestion and nutrient uptake.