Family Cavoliniidae Fischer, 1883
Cavoliniidae is composed of three subfamilies: Clioinae (four genera) Cuvierininae (one genus) and the Cavoliniinae (three genera).
The shell microstructure of Cavoliniidae is a spiral structure whereas in Limacinidae this structure may be found in their protoconch shell (Rampal, 1977).
In the Cavoliniidae the shell coiling is lost, primitive forms (Creseis) have straight tubular shells, the higher evolved genera, such as Clio, have a pyramidal shell or a shell composed of two valve-like structures (Diacria). The protoconch has a rounded to oval shell and in the juvenile stage it grows out as a tubular shell. During further growth a ventral and dorsal side start to differentiate. The primitive circular shell aperture shape is lost as it becomes oval, triangular and finally slit-like in Cavolinia. Lateral ribs are sometimes prolongated and spines develop. The protoconch, with juvenile shell parts, forms a caudal spine; in the genera Hyalocylis, Styliola, Cuvierina, Diacria, Cavolinia and Diacavolinia, this caudal spine is actively thrown off during the adult stage to increase floating capacity. A periostracum is absent and the sculpture is restricted to growth lines and transversal ribs within some species faint longitudinal lines in some species.
The soft parts in Cavoliniidae, as illustrated for Cavolinia and Creseis, also show secondary bilateral symmetry. The columellar muscle is not fixed to the columella but to the innerside of the protoconch. In species that lose the caudal spine the muscle is fixed to a closing membrane formed in the opening left after throwing off of the caudal shell part. The gonads are apical and the liver, enveloping the gizzard with gizzard plates, is found directly under the neck region. A mantle is present at the ventral side with a well developed mantle gland. Asymmetrical dorsal tentacles are found in the neck region where the seminal groove is also situated. The evaginable penis is lost in the female stage. In living specimens the 'balancers', prolongations of the mantle can be seen tapering behind the swimming animal.
[3 subfamilies]