(Abridged and adapted from relevant wikipedia entries)
A lobster-pot trap is a chamber that is easy to enter, and whose exit is either difficult to find or is obstructed by inward-pointing bristles. Lobster pots are the trapping mechanism in Genlisea, the corkscrew plants. These plants appear to specialise in aquatic protozoa. A Y-shaped modified leaf allows prey to enter but not exit. Inward-pointing hairs force the prey to move in a particular direction. Prey entering the spiral entrance that coils around the upper two arms of the Y are forced to move inexorably towards a stomach in the lower arm of the Y, where they are digested. Prey movement is also thought to be encouraged by water movement through the trap, produced in a similar way to the vacuum in bladder traps, and probably evolutionarily related to it.
Outside of Genlisea, features reminiscent of lobster-pot traps can be seen in Sarracenia psittacina, Darlingtonia californica, and, some horticulturalists argue, Nepenthes aristolochioides.
Genlisea are small herbs, growing from a slender rhizome and bearing two morphologically distinct leaf types - photosynthetic foliage leaves above round and highly modified subterranean leaves used to trap prey. The plants lack roots, although the subterranean traps perform many of the functions, such as anchorage and water absorption, normally performed by roots.
6. Borderline carnivores
To be a fully fledged carnivore, a plant must attract, kill, and digest prey; and it must benefit from absorbing the products of the digestion (mostly amino acids and ammonium ions). To many horticulturalists, these distinctions are a matter of taste. There is a spectrum of carnivory found in plants: from completely non-carnivorous plants like cabbages, to borderline carnivores, to unspecialised and simple traps, like Heliamphora, to extremely specialised and complex traps, like that of the Venus flytrap.
The cause of the problem is that the classification of carnivore is a false grouping together of unrelated plants, and no single definition can possibly encompass them all..
Genlisea violacea unearthed to show subterranean traps. Image created by: Noah Elhardt