Zoology 10th Edition by Stephen Miller,John Harley

Exercise 6

Members of the phylum Porifera include the sponges. Their bodies consist of a body wall with pinacocytes, mesenchyme cells, and choanocytes organized around a central cavity or branching chambers. Choanocytes function in creating water currents and trapping food particles in the water. Large sponges require complexly branched canal systems to provide room for adequate filtering by choanocytes to support the larger body size. Ciliated larval stages are produced by most sponges.
In spite of having distinctive cell types that cooperate in a division of labor, sponges lack tissue-level organization. Why is the absence of tissue-level organization an appropriate description for sponge organization?