Chapter 24, Problem 25A

Summary Introduction

To write:

The term that does not belong in the group

Introduction:

Cnidarians include specialised cells called as cnidocytes, which include organelles called nematocysts (stingers). These cells are present around the mouth and tentacles, helping immobilize prey with toxins contained inside the cells. Spicules are structural elememts found in most sponges. They give structural support and deter predators. 

Explanation of Solution

Cnidarian tentacles are armed with stinging cells known as cnidocytes. Cnidarians get their name from the stringing cells. Cnidocytes include nematocysts. A nematocyst is a capsule that holds a coiled, threadlike tube containing poison and barbs. Spicules are structural elements discovered in most of sponges. They give structural support and deter predators. Large spicules that are seen to the naked eye are called megascleres, whilst smaller, microscopic ones are termed microscleres.

Conclusion

Cnidarians are together because they are all armed with stinging cells called cnidocytes and consist a structure called a nematocyst. Spicules are not part of the group because these are found in sponges.