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.
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.
In Gnetum Gnemon, as described by Lotsy, a mature embryo-sac contains in the upper part a large central vacuole and a peripheral layer of protoplasm, including several nuclei, which take the place of the archegonia of Ephedra; the lower part of the embryo-sac, separated from the upper by a constriction, is full of parenchyma.
In division a vertical constriction divides a zooid into two equal or unequal parts, and the several parts of the two corals thus produced are severally derived from the corresponding parts of the dividing corallum.
The forward portion of the anterior coelom shared in the constriction and elongation of the preoral lobe; but its hinder portion was dragged up along with the water-pore and formed a canal lying moccth ive along the outer wall (the gerei tatpore parietal canal).