The box has the handles/'pontoons' on the side to provide air pockets for flotation. The sides are completely vented to allow for water to flow through. The top, in fact, has vents in it as well. The V-Shaped divider in this one is an insert. You put the divider in, then place momma in the device on top of the divider. In theory the fish fall through the divider when she gives birth, keeping them from her. When she's done giving birth you remove her from the trap, then remove the divider and leave the device floating to provide a safe haven for the fry.

Now, that's how it works in theory... In practice some of the other posts about these breeders sound spot on. I just had an experience where a platy placed in a breeding trap scarfed her own fry because they do have a tendancy to find their way up through the divider, once there they're in close quarters with a larger fish who has no maternal instinct as we know it... she sees something taking up her space and she's hungry.

Another poster mentioned that the surface area seems low, and if you place the container the way it's displayed I can see where that would be an issue... however, if you were to place the breeder in the water and tip it so that most of the air can escape from the pockets provided by the handles it will sink down nicely to the surface, if you have a strong filter current you can even get it placed so it's completely submerged by the flow from the filter but still remains at the surface. This would provide significantly more room for the dropping fish than letting it float where the air pockets would form normally and increase comfort... somewhat.

I currently have two molly fry in one of these breeding traps and they have been there for a few days at this point. I placed a piece of hair algae in there with them and it's a lot easier to make sure they have access to a food supply in a segregated area of the aquarium. I'll be able to report success or failure in keeping them in there within a few weeks. On the other hand, I refer back to the instance I just had where I know that a platy female dropped in my breeder because I saw at least one fry overnight, but when I returned the following morning he no longer existed.