Question

Which of the following vertebrate organ systems does not open directly to the external environment?
A. Digestive system
B. Circulatory system
C. Excretory system
D. Respiratory system

Answer 1

Another name for that system which does not open directly to the external environment is the cardiovascular system which allows the blood to circulate.

Complete answer:
If we look at options, option A is Digestive system. It involves the breakdown of complex food into simpler and smaller components and then it is absorbed and assimilated into the body. The main organs are mouth, esophagus, stomach, small intestine, large intestine, rectum and anus. In the questions, the vertebrate organ system is asked which does not open directly to the external environment but the digestive system opens directly to the external environment as it includes the mouth. Therefore, this option is incorrect and hence it gets eliminated.
Moving to option B i.e., Circulatory system. It is also a cardiovascular system which allows the blood to circulate and transport nutrients, carbon dioxide, oxygen, hormones and blood cells from the cells to the cells in the body in order to provide nourishment and help the body in order to fight diseases. This system does not involve any external organ due to which it does not open directly to the external environment and that is why this option is the correct one.
Moving to option C i.e., Excretory system. This system involves the removal of excess and unnecessary materials from the body fluid of an organism. This excretion is to maintain the internal chemical homeostasis and also to prevent damage to the body. Organs of the excretory system are skin, liver, large intestine, lungs and kidneys. In the questions, the vertebrate organ system is asked which does not open directly to the external environment but excretory system opens directly to the external environment. Therefore, this option is incorrect and hence it gets eliminated.
Last option D is Respiratory system. This system includes those organs and structures which help us to breathe and the organs are Nose, Mouth, Trachea, Pharynx, Larynx, Bronchi, Lungs. In the questions, the vertebrate organ system is asked which does not open directly to the external environment but the respiratory system opens directly to the external environment as it includes nose and mouth. Therefore, this option is incorrect and hence it gets eliminated.

Hence, the correct answer is option (B).

Note: From all the given options, Digestive system, Respiratory system and Excretory system open directly to the external environment as they include external organs to work properly but in the circulatory system only internal organs are required to function due to which it does not open directly to the external environment.