The division in all cases takes place by constriction, or by a simultaneous splitting along an equatorial plane.
Often the head is retractile, and a constriction of flexible cuticle distal to it is termed a neck: in Philodinaceae there are a series of thin flexible rings which permit both distal and proximal ends to be telescoped into the middle; and in Taphrocampa, regular constrictions of the whole bodywall give an appearance of metemeric segmentation to the body.
There is also felt a sense of constriction in the pharynx, due to the action of the drug on its muscular fibres.
Characters: Barbels, three to six pairs; pharyngeal teeth in one row, in moderate number; anterior part of the air-bladder divided into a right and left chamber, separated by a constriction, and enclosed in a bony capsule, the posterior part free or absent.
D, First constriction of the Aurelia scyphistoma to form the pile of ephyrae or young medusae.