If an organism is supported only by cartilage, does it have an endoskeleton?
From wikipedia


  An endoskeleton (From Greek ἔνδον, éndon = "within", "inner" + σκελετός, skeletos = "skeleton") is an internal support structure of an animal, composed of mineralized tissue.


Cartilage is a mineralized tissue so it counts as a skeleton from this definition. A bit further in the wikipedia article it says


  The vertebrate endoskeleton is basically made up of two types of tissues (bone and cartilage)