Final answer:
Petromyzon, or lampreys, differ from Scoliodon, or sharks, by having a circular, jawless mouth and by lacking scales. The correct answer, which demonstrates these differences, is D. (3) and (4).
Step-by-step explanation:
The question asks about differences between Petromyzon and Scoliodon in a biological context. Petromyzon, commonly known as lampreys, belong to the class Petromyzontida and are jawless fishes, which reflects one of their main characteristics compared to Scoliodon, a shark that is a member of the subclass Elasmobranchii within the class Chondrichthyes. Lampreys differ from sharks in having a circular, jawless mouth that is used for their suction feeding method. Their mouth structure contrasts with sharks that have jaws. Furthermore, Scoliodon possesses scales, characteristic of many sharks, while Petromyzon does not have scales. Therefore, the correct options regarding the features that Petromyzon has compared to Scoliodon are circular mouth and absence of scales.
In terms of the options provided:
(1) Jawed mouth - Incorrect, as Petromyzon does not have jaws
(2) Cranium - Both Petromyzon and Scoliodon have craniums, so this is not a differentiating characteristic.
(3) Circular mouth - Correct, as this feature is specific to Petromyzon and other jawless fishes.
(4) Scales - Incorrect, since Petromyzon does not have scales.
The correct option in the final answer is D. (3) and (4), meaning that Petromyzon differs from Scoliodon in having a circular mouth and lacking scales.