The Shell

The Shell is a boss added in The Binding of Isaac: Repentance that can appear in.

Behavior
The Shell always spawns in a pair consisting of 4 body segments and a head segment each. They are always fought in a double-sized room.
 * Flies around the room diagonally, bouncing off of walls while dealing contact damage; behaving identically to The Hollow.
 * Unlike the Hollow, if Isaac is close to the vicinity of its head, it veers slightly in Isaac's direction.
 * Normally invincible, but bombs and explosives can destroy its bony exterior (after 15 damage per segment), making its segments vulnerable. There are always one or two Bomb Grimaces that dispense Throwable Bombs for Isaac to use against The Shell.
 * When the armor on their body is destroyed, the exposed flesh segments will spawn Fly Bombs if the segment is at the very end, or shoot a volley of three shots at Isaac if the segment is in the middle.
 * The Shell moves slower if there are exposed flesh segments attached to its head.
 * If a group of conjoined flesh segments have been detached from the head, they chase directly after Isaac at a very slow pace and continue to attack normally.
 * If a single flesh segment has been completely detached, it becomes stationary but continues to spawn Fly Bombs.
 * The Shell cannot split into smaller copies; when split down the middle, the side containing 3 segments with at least one of end segments having armor turns into a new copy of the boss with one of the armored body segments turning into a head segment.
 * Flesh body segments on the shorter side detach and behave normally but armored body segments are automatically destroyed.
 * The Shell's head cannot be damaged or destroyed and dies once it loses all its segments.

Trivia

 * They appear to be a bony variation of The Hollow.
 * They appear to be the posthumous version of the Tuff Twins. This makes them one of four boss pairs to have their regular and posthumous version in the same chapter, the others being Gemini having Blighted Ovum in Chapter 1, Chub having Carrion Queen in Chapter 2 and Polycephalus having The Stain in Chapter 2.