In which group of animals do we find a mixed heart where oxygenated and deoxygenated blood can partially mix in the chambers?

Introduction / Context:
The term mixed heart refers to a heart in which oxygenated and deoxygenated blood are not completely separated and can partially mix within the chambers. Understanding which animal groups have such hearts is important in basic zoology and helps explain differences in circulatory efficiency among vertebrates.

Given Data / Assumptions:

The options list reptiles, nematodes, birds, fishes and mammals. The question uses the standard school level concept of a mixed heart. We focus on the typical pattern within each group, not rare exceptions.

Concept / Approach:
A mixed heart is usually associated with a three chambered design, where two atria and a single ventricle allow some mixing of oxygen rich and oxygen poor blood. Amphibians and most reptiles have this type of heart. Reptiles, except for crocodilians, possess a heart with two atria and a partially divided ventricle, leading to incomplete separation of oxygenated and deoxygenated blood. Birds and mammals have four chambered hearts with complete separation, and fishes have a two chambered heart with a single circulation pathway, not described as mixed in the same way. Nematodes are simple invertebrates with a different type of body fluid circulation, not the vertebrate heart pattern.

Step-by-Step Solution:
Step 1: Recall that fishes generally have a two chambered heart with a single atrium and a single ventricle. Step 2: Amphibians and most reptiles have a three chambered heart which can mix oxygenated and deoxygenated blood in the ventricle. Step 3: Birds and mammals have a four chambered heart, which completely separates the two types of blood and supports high metabolic rates. Step 4: Among the options, reptiles are the vertebrate group most strongly associated with a mixed heart in basic zoology discussions. Step 5: Therefore, reptiles are chosen as the correct answer, while remembering that crocodiles form an advanced exception with a four chambered heart.

Verification / Alternative check:
A quick check is to think about body temperature regulation. Birds and mammals are warm blooded and require very efficient separation of oxygenated and deoxygenated blood, so they must have four chambered hearts. Fishes have a single circulation through gills and body and are not typically described with the mixed heart terminology in textbooks. Reptiles, along with amphibians, are often discussed as cold blooded animals where mixing in the heart is one reason for lower circulatory efficiency, which fits the phrase mixed heart well.

Why Other Options Are Wrong:
Nematodes are roundworms without a true vertebrate type heart. Birds and mammals have four chambered hearts with complete separation, not mixed hearts. Fishes have a two chambered heart that sends deoxygenated blood to the gills and then to the body in a single loop, so oxygenated and deoxygenated blood are not mixing in the heart in the same way as in three chambered hearts. Therefore, reptiles are the only correct choice from the list for this particular term.

Common Pitfalls:
Students sometimes confuse amphibians and reptiles because both have three chambered hearts, and some questions specifically mention amphibians. When amphibians are not listed, examine the options carefully and choose reptiles, remembering that both groups share the concept of a mixed heart. Another pitfall is to think that cold blooded always means mixed heart, which is not strictly correct for fishes. Always link the term mixed heart to the idea of a three chambered heart in amphibians and most reptiles.

Final Answer:
Animals with a mixed heart among the options given are found in the group of reptiles.