Final answer:
Acorn worms have a complete digestive system with a separate mouth and anus, which allows for efficient one-way digestion and specialized organ development. This is an advanced form of digestion compared to the early invertebrates' single-opening system or the lack of digestive system in parasites like tapeworms.
Step-by-step explanation:
Type of Digestive Tract in Acorn Worms
Acorn worms, which are part of the phylum Hemichordata, have a complete digestive system. This system is an advanced form of digestion, consisting of a continuous alimentary canal with a separate mouth and anus. This allows food to move through their body in one direction only, improving digestive efficiency. Different specialized regions of the digestive tract are developed for various functions. For instance, once ingested, food moves through the esophagus, is stored in the crop, and then passes into the gizzard where it is broken down. Following this, the food enters the intestine where nutrients are absorbed, and waste is excreted through the anus.
Early invertebrates had an incomplete digestive system with just one opening serving as both mouth and anus, which is less efficient. However, the evolution of a complete digestive system, like that of the acorn worms, allows for ongoing ingestion during digestion and waste elimination, leading to the development of specialized digestive organs.
The digestion in acorn worms starts with the ingestion of food through the mouth, then it is processed in the digestive tract and finalizes with the excretion of waste, which indicates the presence of a complete digestive system, much like that found in earthworms, as detailed in the provided references. In comparison, flatworms and tapeworms have very different types of digestive systems. Flatworms have a gastrovascular cavity with a single opening serving as both mouth and anus, while tapeworms do not have a digestive system at all, as they absorb nutrients directly from their host's digestive system.