In Petromyzon :

Question

In Petromyzon :

Accepted Answer

The correct option is 4: A Lymphatic system is present.

The statement that *Petromyzon*, a lamprey and a representative of the most basal group of extant vertebrates (Cyclostomata/Agnatha), possesses a lymphatic system is factually correct. The lymphatic system is a vital component of the circulatory and immune systems, and its presence in lampreys reflects its evolutionary antiquity, as the system is found in all vertebrates. This system is a network of vessels that is separate from the blood vessels; its essential function is to collect excess interstitial fluid (which is then called lymph) that has leaked from blood capillaries into the body's tissues and return it to the bloodstream. This drainage is critical for maintaining fluid balance (homeostasis). Although the lymphatic system in lampreys is considered simpler than in higher vertebrates, for instance often lacking the well-defined, encapsulated lymph nodes of mammals, it is nonetheless a complete and functional accessory circulation essential for immunity and fluid management.

The incorrect option is 1: Liver is tetralobed. The term 'tetralobed' means having four distinct lobes, a characteristic that describes the liver morphology in many mammalian species, such as mice or rabbits. The liver of *Petromyzon* is not divided in this specific four-lobed manner. While the lamprey liver undergoes unique developmental changes—specifically the atrophy or disappearance of the bile duct in the adult stage, which transitions the adult to a non-feeding life phase—the organ itself is structurally simple and is generally regarded as a single, compact mass, not distinctly segmented into four lobes.

The incorrect option is 2: Renal portal system is present. This is factually incorrect because *Petromyzon* is a jawless fish, and one of the significant distinguishing features of this basal vertebrate group (Agnatha) is the *absence* of a true renal portal system. The renal portal system is a network of veins that carries blood from the posterior parts of the body (like the tail) directly to the kidneys before it continues to the general circulation. This system is a characteristic found in the majority of jawed vertebrates (Gnathostomes), including most fishes, amphibians, and reptiles, but its absence in the lamprey is a primitive trait that highlights its ancestral position in vertebrate evolution.

The incorrect option is 3: Hepatic portal vein is absent. This statement is incorrect as the hepatic portal system, including the hepatic portal vein, is a fundamental and highly conserved feature of the circulatory system present in *all* vertebrates. The hepatic portal vein is responsible for collecting blood rich in newly absorbed nutrients and sometimes toxins from the digestive organs (the intestine) and shunting it directly to the liver. This anatomical arrangement ensures that the liver can process, detoxify, and regulate the concentration of these substances before they are distributed to the systemic circulation, a function so vital that it must exist even in the most primitive vertebrates like *Petromyzon*.