Final answer:
Having a digestive system with only one opening means an organism can't ingest new food while digesting, and the digestive process is less specialized and efficient compared to a system with separate mouth and anus.
Step-by-step explanation:
The disadvantages of having a digestive system with only one opening are primarily tied to the limitations in efficiency and specialization. An organism with a gastrovascular cavity, which has a single opening for both ingestion and excretion, must finish digesting and assimilating food before it can take in more food. This process is less efficient compared to organisms with a complete digestive system, where food moves in one direction from mouth to anus, allowing for continuous feeding and digestion. In a complete system, different parts of the alimentary canal can become specialized to perform different digestive functions, thus improving metabolic efficiency and health.
In contrast, organisms with a single-opening digestive system, such as flatworms, cannot eat while they are still digesting food, as they would risk expelling undigested food along with waste material. This potentially reduces the amount of nutrients they can absorb in a given time frame. Additionally, the lack of specialization in their digestive tract means they are less able to adapt to different types of food or to engage in complex digestive processes that higher animals with complete digestive systems can.