Spineless, yes, but still amazingly well adapted and the most diverse and prolific category of organism.  They can be insects, good and bad (and dinner, don’t forget) that spend all their time on land or part of the time in water.  They can also be crawfish, looking just like mini freshwater lobster, or freshwater mussels.  Some are beautiful, like the dragonflies, and some not so much, like leeches.