Final answer:
Finding an internal organ such as a heart or a brain in a sponge would be a great surprise because sponges are simple multicellular organisms without true tissues or organs. They have a cellular level of organization with a body full of pores allowing water circulation and an endoskeleton comprised mainly of spicules.
Step-by-step explanation:
It would be a great surprise if you found an internal organ, like a heart or a brain, in the body of a sponge, as sponges are much simpler organisms. They do not have true tissues or organs, and their bodies are categorized by a cellular level of organization. Most notably, sponges are multicellular organisms with bodies full of pores (hence the phylum name Porifera), which facilitate the circulation of water through them. The structure of a sponge includes a spongocoel, osculum, and an endoskeleton made of spicules, which can be comprised of silica, calcium carbonate, or spongin.
The presence of endosymbionts like green algae or cyanobacteria inside sponges wouldn't be surprising, nor would the discovery of carnivorous sponges. Similarly, finding spicules as part of their support structure is to be expected, given that these are common among various sponge classes. However, encountering advanced organs or tissue-level structures would indeed be shocking, considering the simple organization level of sponges.