Types of protoconch:
- Type A: angle of the protoconch with the shell axis ranging from 90° to c. 110°. A-I is planispiral or planorbid, with all protoconch whorls mainly in one plane. A-II is helicoid, with a clearly protruding nucleus. In some cases the first teleoconch whorl overlaps the protoconch, but even in those cases the topwhorl can still be seen.
- Type B: angle of the protoconch with the axis of the shell ranging from 120° to c. 160°. Here the topwhorl usually cannot be seen because it is hidden within the first teleoconch whorl; the rest of the embryonic whorls is visible, however.
- Type C: angle almost 180°. Here the topwhorls have almost disappeared into the first teleoconch whorl.
The types B and C are usually referred to as 'intorted'. It should be emphasized that there is very little intraspecific variation in the structure of the protoconch, which is often diagnostic.

This family consists mostly of small to minute shells of varied form, some many-whorled, tall and slender, others ovate and rissoid-like, but differing in that the protoconch is sinistral, partly immersed, and set at an angle to the dextral adult whorls. Although carnivorous, the radula is absent, but they have an oral sucker and a buccal stylet that pierces the body of their host, and a sort of buccal pump extracts the victims body juices. They are of world-wide distribution, are exceedingly prolific in species; some roam freely in the substratum, but most favour a particular organism as host, which may be a polychaete worm, an echinoderm, a coelenterate, or a bivalve mollusc.
The pyramidellids are now considered to be nearer related to the opisthobranchs, rather than to the prosobranchs, where they were formerly assigned.