The spiral shell, which is primitively still present, shows a strong tendency toward shortening of the spire and toward widening of the aperture, hence the spire is in most cases completely enclosed by the body whorl; moreover, the shell becomes surrounded by the mantle and becomes thin, and is finally completely lost. The mantle cavity shifts to the right from its anterior position and expands posteriorly, thereupon, along with the shell, the mantle also becomes reduced. The initially small, leaf-shaped gill shifts out of the mantle cavity and may become considerably enlarged, although often it is completely lost and secondary gills may be formed in the anal area, on the sides of the dorsum, or on the underside of the notum margin. An operculum is retained only in actaeonids and spiratellids. The buccal mass contains in most cases 2 jaw plates and a radula, the plates of which are sometimes very numerous, sometimes greatly reduced, and finally only single longitudinal row is developed. In the nervous system, corresponding to the displacement of the gill with the supraintestinal ganglion, the visceral commissure is only slightly or not crossed and the ganglia in most cases come closer to each other. The heart turns with the gill; its auricle comes to lie on the right side and then posterior to the ventricle. The gonad is always hermaphroditic, at the oviduct mucous and albumen glands are usually developed, as well as 1 or 2 seminal vesicles, at the sperm duct a more or less distinct prostate; with the exception of the actaeonids, the penis is retractile, its opening is primitively distinct from the genital opening, with which it is connected only by a ciliated groove, although the two openings in most cases come close together until they unite, sometimes a third opening is also present.