Question

Heart lacks sinus venosus in
A. Fishes
B. Mammals
C. Amphibians
D. Echinoderms

Answer 1

Hint: The heart is a muscular organ which consists of four chambers: two atria and two ventricles. Its function is to deliver the oxygenated blood to tissues. Human heart is about the size of a fist which pumps blood through the network of arteries and veins called the cardiovascular system.

Complete answer:
In amphibians, a membrane called sinus venosus is present in the cavity of the heart between two vena cavae in the atrium region. This membrane develops to form a soft, smooth part known as sinus venarum in the right ventricle in order to form as SA node and coronary sinus.
In fishes, sinus venosus is present. Its function it to receive blood from from all over its body and pass it on to the heart through atrium
The large quadrangular cavity that precedes the right atrium is known as sinus venosus. It is found in an enlarged region between the vena cava and the right atrium. It is developed by the unification of large systemic veins. In lower vertebrates, the sinus venosus acts as a passage through which the venous blood from all over the body enters into the heart.
In mammals, the thin walls of the sinus venosus found distinctively in the embryonic heart where it exists between the two venae cavae (i.e. Superior and Inferior vena cava) and right atrium. However, the sinus venosus shrinks in size and disappears in adulthood as it has been incorporated into the wall of the right atrium to create a soft part known as sinus venarum.
Certain adult birds and mammals lack the presence of a sinus venosus, precaval and postcaval veins in their heart. Thus the hearts of mammals lack sinus venosus.

Hence, the correct answer is option (B).

Note: One of the distinguished features in mammals is the structure of the brain. The brain of mammals is relatively large and has a thin layered covering called the neocortex which is responsible to perform many complex brain functions. The presence of neocortex is found only in mammals and not seen in birds or in reptiles.