In most gastropods (snails and slugs) all openings of the body (except the pores of some mucus glands) are located in or near the head. Especially in slugs, the pneumostome (breathing pore), genital opening, anus and the opening of the kidney are very close to each other and only a short distance away from the mouth.
In terrestrial pulmonate gastropods the eggs are expelled thru the genital opening, which is located on the right side of the head (if the individual is dextral, otherwise, on the left side). The series of pictures below show the slug Arion subfuscus expelling one egg.
Arion subfuscus laying an egg. White arrow: pneumostome; yellow arrow: genital opening.
The developmental process that brings the pneumostome, anus and the opening of the kidney to the front of the body is known as torsion. Torsion, which occurs early in larval development, is defined as the twisting of the body behind the head, including the visceral mass, mantle and mantle cavity, 180° counterclockwise.
Previous post in this series was about how a slug breathes.