What is It?

This creature was found chewing on a hibiscus leaf. It is nearly oval, light green with whitish streaks and looks like half a minature watermelon. It doesn't have legs as such but moves by ripples. In place of feet it has single brown claws. No head or eyes show on this creature because they are hidden inside a vent on its underside. One can see two large brown teeth inside the vent. When turned upside down it will, sometimes, stick its head out of the vent showing two large eyes. The animal is smooth to the touch and firm. It is quite glossy. Click on photos to enlarge.