So, this is a commission done by the awesome  plasticle of my Cacturne predator character, Serana, whom also has a male form named Seran.
Given that the only discernible difference between the two gender forms are a set of growths on their arms being there or not and what they're packing between their legs, this applies to both of them.

The general design behind the inner mouth is a tongueless plant fleshy maw in the shape of a barrel cactus, in a way. The throat and stomach also follow this design, but the stomach moreso than the throat. At the back of their maw is a sphincter-like opening that can open and close to accept food or such. It's also a representation of their plant anatomy as their throat can be rather tight.

Along the color pattern, the closest internally that one gets to their core, the more their internal body color shifts to a dark green, hence why the upper part of their stomach holds that dark green hue.

The outer mouth is exactly the same as a normal Cacturne's. What makes Serana/Seran unique is that they can stretch these mouth holes to accommodate prey. Stretching their maw wide enough will cause tears to form on the centermost flesh between the mouth holes, leaving to a wider mouth to accept prey, although the flesh in-between the holes can be pushed aside as well. These tears heal rather quickly.

The entirety of the inner mouth, throat, stomach, and so on is lined with soft, spine-like protrusions that act almost like a sensory array, able to taste, shift, and overall add an extra level of feeling to the experience for them.

Their insides are slick to the touch, and the taste of it comes off as a variation of cactus water, yet this moisture does not exist outside of their body. It is an internally produced substance that is not dried out by intense heat, and provides lubrication for prey to allow easier swallowing and such. The stomach and entire digestive system holds this quality.

The inner flesh of the mouth is capable of a full range of movement, essentially able to shift and bulge accordingly to turn prey around, roll them around, or so forth.

Serana/Seran are capable of endosoma and fatal vore, but with their tendencies as a predator, they are predominantly an endosoma pred, moreso a healer pred than a taker pred, usually holding prey inside of their stomach as a way to shield them from intense weather should the situation permit in addition to provide a warm, safe place for weary travelers or whomever they come across.

Should they choose to digest their prey, the walls of the stomach contract strongly as a thick, goopy substance oozes into the chamber from the walls. The walls will contract until the prey cannot move and all the free air has been pumped out of the chamber, to the point where it's not unlikely for bones, particularly ribs, to be broken from the force, while the thick oozing substance completely fills the chamber, submerging the prey within it. The process of digestion is slower than a fleshy pred, but the ooze itself is comprised of various chemicals that acts as a knockout cocktail, essentially knocking the prey into unconsciousness before the digestive fluid or their stomach kills them, so the act is a peaceful sleep that one simply drifts off into. Granted, they don't know reformation, so this only applies to prey that they must hunt to survive or for willing prey whom ask for it.

Serana/Seran are gentle preds for the most part. Not only being healing preds, they have a tendency to take their time with prey, making sure they're comfortable especially during endosoma situations.

Left: Torn open mouth. Fully stretched open
Middle: Normal open mouth. Just holding it open.
Right: Stomach view.

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Art:
Serana/Seran © Me