Final answer:
The polyp structures that determine whether it is a gastrozooid or a gonozooid in Obelia are differentiated by their functions: gastrozooids have a mouth and tentacles for feeding, while gonozooids lack these structures and instead produce reproductive buds for sexual reproduction.
Step-by-step explanation:
The structures that determine whether a polyp of Obelia is a gastrozooid or a gonozooid are related to their functions within the colonial organism. A gastrozooid, also referred to as a feeding polyp or hydranth, is adapted for capturing prey and feeding, while a gonozooid, also known as a gonangium, is adapted for asexual budding and the production of sexual medusae. Gastrozooids can be identified by the presence of a mouth opening surrounded by tentacles at the oral end, which is designed to facilitate feeding. In contrast, gonozooids are characterized by their ability to produce reproductive buds that break off and mature into free-swimming medusae, which are part of the organism's sexual reproduction process.
Furthermore, while all polyps in Obelia are connected through a shared digestive cavity known as a coenosarc, it is the specific adaptation of the polyps' structures to either feeding or reproductive functions that differentiates them. The gonozooids' lack of a mouth and tentacles, coupled with the formation of reproductive buds, are key identifying features separate from the structure of gastrozooids.