The telson is the last segment of the body. In this case, it will be flatened with three bumps on the dorsal surface.
Most species of Haptosquilla do not get very large - under two inches - and they are fairly long-lived - at least three or four years. In my expereince, if reasonably well fed, they are not particularly aggressive towards fish larger than themselves. Like I said earlier, they tend to remain in their cavity and snatch prey that come near the entrance. When I have kept them with fish, the fish generally learned to stay away from the cavity and all was fine.
Roy