Which of the following is TRUE of an open circulatory system, but NOT TRUE of a closed circulatory system?

a. water is drawn into the mantle cavity to provide oxygen to body tissues.
b. lungs branch into small tubules to provide oxygen to tissues.
c. wastes are eliminated directly to the environment from tissues
d. body tissues are bathed directly in fluid containing oxygen
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The correct answer is:

a. water is drawn into the mantle cavity to provide oxygen to body tissues.

In an open circulatory system, there is a fluid called hemolymph that is not contained within vessels and directly bathes the body tissues. This fluid contains oxygen and is circulated through a series of interconnected spaces and sinuses. In some organisms with an open circulatory system, such as mollusks, water may be drawn into the mantle cavity to supply oxygen to the hemolymph.

On the other hand, in a closed circulatory system, blood is contained within vessels and pumped by a heart. Oxygen is typically transported to tissues by blood vessels, such as capillaries, which allow exchange of materials between the blood and the surrounding tissues. Waste products are usually eliminated from the tissues through other mechanisms, such as diffusion or filtration, rather than being directly released to the environment.

To determine the correct answer, let's first understand the difference between an open circulatory system and a closed circulatory system.

In an open circulatory system, the fluid (usually called hemolymph) that carries nutrients and waste products is not enclosed within blood vessels. Instead, it directly bathes the body tissues. This type of system is found in some invertebrates, such as insects and most mollusks.

On the other hand, a closed circulatory system is characterized by the presence of blood vessels that transport blood containing oxygen, nutrients, and waste products. The blood is confined within these vessels, which helps in directing the flow and regulation of circulation. Closed circulatory systems are found in most vertebrates, including humans.

Now, let's evaluate the options provided to determine which one is true for an open circulatory system but not for a closed circulatory system:

a. Water is drawn into the mantle cavity to provide oxygen to body tissues.
This statement refers to the respiration process in mollusks, specifically those with gills, like clams or mussels. They use the mantle cavity to draw in water that passes over the gills, extracting oxygen from it. This method is only relevant to organisms with an open circulatory system, making this statement true for such a system but not for a closed circulatory system.

b. Lungs branch into small tubules to provide oxygen to tissues.
This option describes the respiratory structure found in some insects, such as grasshoppers. They have a network of tiny tubes called tracheae that branch throughout their body and directly supply oxygen to tissues. However, this feature is not related to the circulatory system and is not unique to an open circulatory system. So, this statement is not true for an open circulatory system.

c. Wastes are eliminated directly to the environment from tissues.
In an open circulatory system, waste products, such as metabolic byproducts, are not transported within blood vessels. Instead, they can be eliminated directly into the surrounding environment through diffusion or other excretory mechanisms. Thus, this statement is true for an open circulatory system but not for a closed circulatory system where waste products are typically transported and eliminated through specific organs like the kidneys.

d. Body tissues are bathed directly in fluid containing oxygen.
This statement describes the characteristics of an open circulatory system, where the hemolymph directly comes into contact with the body tissues, supplying them with oxygen. In a closed circulatory system, the blood is contained within vessels and doesn't directly bathe the tissues. Therefore, this statement is not true for a closed circulatory system.

Based on the above analysis, the answer is:
The correct option that is TRUE for an open circulatory system but NOT TRUE for a closed circulatory system is (c) wastes are eliminated directly to the environment from tissues.

By process of elimination, I would say that the answer is A. B, C, D are all true with regard to a closed system.